Trim release docker image

[tomuben]

Background

For template flavors, we provide the flavor_customization step, which allows customers to install custom packages. The Docker file of the release build-step then copies the content from language_deps and flavor_customization.
It can be observed, that this has no impact on the final docker image size as long as the flavor_customozation step is empty. But whenever a single package is added to the flavor_customization step, the size of the Docker image doubles.
This has a big impact for the user, as well for the CI of some of our projects (e.g. https://github.com/exasol/notebook-connector/).
The reason for the copy statements of the language_deps build step was to avoid possible changes of the required packages during the flavor_customization build-step. However. we can assume that these changes won't happen.

It is actually enough to just copy the content of the flavor_customization step.

Acceptance Criteria

• Remove the copy statements in the Dockerfile of the release build-steps, which copy the content of the language_deps .

[tkilias]

We probably don't copy the flavor_customization step, but use it as the base image. Furthermore, the same can be said for the flavor_base_deps step in the standard flavors.