Dockerfile updates

[lmctv]

With the enclosed changes we manage to:

1. Use up-to-date ruby interpreter and base image
2. Parametrize the db adapter from the environment, just as it was already happening for the db name and credentials

The use of puma instead of passenger stems from failures in the complex passenger update.

[mjansing]

Thanks for this PR. I updated our Docker-Setup in d25abd07b492322278badf154e8da6c699c80637. I made a lot of changes which you also addresses in this PR (Puma instead of passenger, Env-Vars). The new Setup exclusively uses PostgreSQL as a primary database. I hope you're fine with it.

[lmctv]

In my case, I needed to "recover" a broken iqvoc environment while continuing to support the already in-use mysql database, and then sent the whole working branch as a PR. I can keep my needed changes in a private branch; still, if there is no strong a-priori objection, I'd like to offer:

1. a PR creating and exploiting a parametric database config config/database.yml.multi §
2. a PR introducing a two-steps build, creating a runtime container based on ruby:2.6-slim rather than on ruby:2.6? This would lead to a leaner runtime environment, which IMHO could be at least a bit safer by not sporting a complete code build environment.

§ if this makes sense, b604ca6f264c950dfe47746973292f9852cbf33e should get reverted, or made conditional on the database engine being postgresql