Enhance docker-compose.yml to allow running multiple ReSeeD instances on the same host, clean up superfluous environment variables,

fixes https://gitlab.ruhr-uni-bochum.de/FDM/rdm-system/antleaf-projectmanagement/-/issues/480+

This merge request does the following things:

• Removes various variables from the .env file that offer no tangible benefit but needlessly clutter the .env file and introduce unnecessary potentials pitfalls for admins.
• Removes the deprecated "version" property from docker-compose.yml, which causes a warning every time a ReSeeD instance is started.
• Restrict all port forwardings to the container host's loopback interface instead of exposing them on all network interfaces. This affects both docker-compose.yml and docker-compose.development.yml, but it still allows connecting from the container host to all the "internal" services/ports that are forwarded in docker-compose.development.yml.
• Adds a few new variables in the .env file and in docker-compose.yml to make it possible to run multiple instances of ReSeeD simultanously on the same container host.
• Adds docker-compose.build.yml, which we use for building clean container images (docker-compose.yml is used for running those container images in production, but not to build those images).
• Adds support for IPv6.