Check for update in base images

[bschmalhofer]

In Docker Hub there are usually no automatic rebuilds of Images when a base images changes. But base images might change because there are security fixes. So it is interesting to be at least notified when a base image is updated.

There a several ways how to approach this. Let's try first https://github.com/marketplace/actions/docker-image-update-checker .

[bschmalhofer]

Still trying to get to a working schedule workflow. The action DockerImageUpdateChecker seems to be active, but there are no runs yet.

[bschmalhofer]

Fiddled with .github/workflows/docker_image_update_checker.yml so that it can be started manually. An error message should be written when the base image was updated. Let's wait until such an update occured and see how the output looks like.
The last push of the Perl images were 9 days ago, so it looks like upgrades are not really frequent.

[bschmalhofer]

Also, when rebuilding an image, then dockerfile.snapshot should be used for installing the Perl modules from CPAN.

[bschmalhofer]

The Perl Image was update and the check indicated this:

TODO:

• Indicate that the base image was updated also in the list of recent actions

[bschmalhofer]

When the check for changes in the base images is implemented then there will be fairly frequent rebuilds of Docker images. These rebuilds include the installation of CPAN distributions into the Docker image. Currently most of the CPAN dependencies are installed with their most current versions on CPAN. This is fine for infrequent builds as there the builds are tested before they are tagged and built automatically on Docker Hub. For rebuilding already released versions we want a more stable setup. Specifically we want that the same versions of CPAN distrubutions are installed into the rebuilt image. This can be achieved with cpanfile.snapshot support in Carton.

See also #210.

[bschmalhofer]

TODO:

• expand the matrix to also cover 10_0 and 10_1, including special case for 10_0 that has no target

[bschmalhofer]

This needs to be tested again, especially as there were some changes done for #3673.