Build 2 images with Docker from a single Dockerfile while using different base images

Question

I'm building 2 variants of the final container a full version and the slim version. And both variants build their containers in 2 steps:

1. Get all the requirements for the full/slim version and make a base full/slim container
2. Install the app on the full/slim base container and make the final full/slim container

Both full/slim builds have an identical second step, they just use a different base container. And I would like to generalize the second step so I wouldn't have two Dockerfiles containing the same content with just different FROM field.

So I wonder what would be the best approach to have the last step as a single Dockerfile which would generate 2 images and only switch between 2 different FROM. I was thinking about a couple of different ways doing it:

• Have Dockerfile.template and bash generating 2 bespoke Dockerfiles from the template.
• Have 3 different files. Common logic file and 2 unique Dockerfiles for the full/slim installs which would include the common logic file.
• There is support for multiple FROM in the Dockerfile, but that looks to me catering to slightly different needs (and all google searches are indicating that).

But I wonder, is there any better approach? Or do I miss something?

Answer 1

You could use build-time configuration with ARG.

FROM instructions support variables that are declared by any ARG instructions that occur before the first FROM. An ARG declared before a FROM is outside of a build stage, so it can’t be used in any instruction after a FROM. To use the default value of an ARG declared before the first FROM use an ARG instruction without a value inside of a build stage:

Dockerfile:

ARG imagename
FROM $imagename
RUN echo $imagename

Run with two different base images:

docker build --build-arg imagename=alpine .

outputs:

Step 1/3 : ARG imagename
Step 2/3 : FROM $imagename
latest: Pulling from library/alpine
ff3a5c916c92: Pull complete 
Digest: sha256:e1871801d30885a610511c867de0d6baca7ed4e6a2573d506bbec7fd3b03873f
Status: Downloaded newer image for alpine:latest
 ---> 3fd9065eaf02
Step 3/3 : RUN echo $imagename
 ---> Running in 96b45ef959c3

Removing intermediate container 96b45ef959c3
 ---> 779bfc103e9e
Successfully built 779bfc103e9e

Alternatively:

docker build --build-arg imagename=busybox .

results in:

Step 1/3 : ARG imagename
Step 2/3 : FROM $imagename
latest: Pulling from library/busybox
07a152489297: Pull complete 
Digest: sha256:141c253bc4c3fd0a201d32dc1f493bcf3fff003b6df416dea4f41046e0f37d47
Status: Downloaded newer image for busybox:latest
 ---> 8c811b4aec35
Step 3/3 : RUN echo $imagename
 ---> Running in 6027fe4f5b7b

Removing intermediate container 6027fe4f5b7b
 ---> 28640f123967
Successfully built 28640f123967

See also this blogpost for more ideas.