docker assets: cannot build docker images outside the source tree (i.e. against a cdk.out directory)

The feature to support customizing the docker file name introduced in #5652 will pass the full absolute path of the docker file name to docker build. This happens both for the case where a custom docker file name is specified and when it is not specified (defaults to /full/path/to/Dockerfile).

However, since we are staging the build directory into cdk.out (which can potentially be ported to a different system, e.g. as part of a CI/CD process), we need the name of the custom docker file to be relative and not absolute.

See https://github.com/aws/aws-cdk/pull/5652/files#r366863153

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This is 🐛 Bug Report

[iliapolo]

Hi @MrJonBirch. Let me clarify what exactly is happening.

Like you mentioned, during synth, the ContainerImage validates that the Dockerfile actually exists. After this validation passes, the cdk.out directory is created by copying the necessary files from the asset configuration. Its here where the .dockerignore file is processed, which caused the Dockerfile not to be copied into cdk.out.

During cdk deploy, the cdk cli does not use the original path to the Dockerfile, but rather assumes that the Dockerfile exists inside the cdk.out directory. This behavior is intentional, to make the cdk.out directory self-contained and portable across machines and environments.

You are right that this is actually an undocumented feature/behavior. I can point you to the following issues to get more context, and will make sure we add this to the documentation.

• https://github.com/aws/aws-cdk/pull/4104
• https://github.com/aws/aws-cdk/issues/4450

Hope this helps, and thanks!

[mimozell]

@iliapolo thanks for the explanation above. I haven’t seen anything like it in the documentation. It helped me focus on cdk.out for the problem, but I wasn’t able to solve mine with just that information. My .dockerignore file looks like this:

**
!build/libs/*.jar

so I don’t want to copy anything else over to the docker context besides the jar, but it is actually not copied over to cdk.out.

I have tried different variations to get the jar copied over, but nothing worked besides moving the jar to the root of the project instead of having it in a directory which is not realistic. Because of this the COPY command in the Dockerfile fails.

Any ideas how I could get the jar copied over so docker build completes fine?