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Now that were environment variables,

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which we set up at runtime

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with the Docker run command,

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and they can be very useful as you saw.

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Now what are build-time arguments?

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With those, we can actually plug in different values

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into our Dockerfile,

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or into our image when we build that image

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without having to hard code these values

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into the Dockerfile.

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And again, one example in this example here

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could be our port number.

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Yes, we're now using an environment variable

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and therefore this is quite flexible.

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But we actually still have one, not really a problem.

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But one area of improvement, I would say.

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Our default value is still hard coded to 80.

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That might make sense

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and might not be a problem at all.

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But maybe we want to make this default value

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flexible as well.

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So that when we build the image,

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we can actually build the image based on

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one and the same unchanged the Dockerfile multiple times

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with different default values.

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And that's something we can achieve

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with build-time arguments.

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For that, we could add an argument.

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And again, you can add this anywhere, basically,

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I will now add it here and simply give that argument a name.

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And I will name it, "default_port."

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You can also assign a default value here again,

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this time with the equal sign, for example,

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like this to again, set it to 80.

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Now, this argument cannot be used in your code here,

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it's not available there, you can only use it

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in your Dockerfile.

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And even there, you can't use it on every instruction.

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You cannot use it on command, for example,

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because that's a runtime command in the end,

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which is executed when the container starts.

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But you can use your arg on all the other instructions.

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And therefore we can also use it on the env instruction,

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for example, to set the default value

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for this environment variable

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to default_port like this.

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Again, with a $ sign in front of it to tell Docker

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that we're referring to some argument

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or environment variable with this word here.

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So now with that, it's kind of inception.

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I'm aware of that.

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But now with that,

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we can set a dynamic argument which then is set

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as a default value for the dynamic environment variable.

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Yes, but now we can build our image here

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with a name of feedback-node and then a tag

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of let's say, web-app.

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And they offer we wanna use port 80.

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And hence, we don't have to specify anything,

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because that's actually the default for our argument here.

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So we can just build it like this.

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But there after, we can build this image again, now,

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let's say for the dev tag for development,

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and now here, we can set a concrete value for default port

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by adding --build-arg,

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and then setting default port equal to 8000.

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Like this, for example.

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And now we're again building an image

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on a different tag with a different arg value.

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There after this value is locked in.

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But now we got two images based on the same Dockerfile,

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where we didn't need to change any code.

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And yet, we're using a different port number,

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which is then used as a default

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for the environment variable.

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And that's just one example.

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But it can be useful to be able

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to lock certain values in when you build an image

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and to be able to build different images

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in a flexible way without having

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to change the Dockerfile all the time.

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So for now, hit Enter, this is being built.

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Enter after we have this image available as well.

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Now one note about where you specify args and envs.

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As you might have noticed,

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it always rerun npm install for me here.

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You might of course wanna place arg and env

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in a place where you then need it thereafter,

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and not place it right at the beginning

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of your Dockerfile because these instructions

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just like all other instructions,

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add layers to your Dockerfile.

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And hence, when something changes about them,

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all subsequent layers are rebuilt, are re-executed.

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So all subsequent instructions are re-executed.

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And I don't wanna execute npm install again all the time.

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Just because I passed in a different default port argument.

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It's not relevant to this command.

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So this command shouldn't rerun just because I did that.

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That's just an extra note on where to place these things.

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But with that, that's it for arguments and environments.

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That's how we can use them.

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And why they might be useful in certain applications.

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We're going to see more of that,

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throughout the course, of course.