add USER directive to generated Dockerfile

[adk-swisstopo]

Most (all?) the Dockerfiles that are generated do not contain a USER directive. This means the code within the container runs as root by default. This does not seem desirable. Please consider add a USER directive to the generated Dockerfiles such that the containers have a default non-root user.

If you think it makes sense and can point me in the right direction about how to implement this, I may be able to send a PR for this. CONTRIBUTING.md says to "check the chapter Adding and modifying containers" but I cannot find that chapter anywhere in this repository.

[adk-swisstopo]

I think the change involves setting a sensible default for entrypoint_user. AFAICT there are only two packages using that parameter right now.

[alexandrevicenzi]

This is something on our radar, but it makes more sense for application images only (e.g mariadb, nginx, etc), which are a few.

If you are using one of the base images, or a development image, we expect that you extend this image and make appropriate changes, including user/group ownership.