feat: hocon reads env vars

[bertbesser]

what it's about

Makes hocon incorporate environment variables into the configuration. This is a common scenario in e.g. containerized runtimes such as Kubeflow pipelines.

most important points of this PR

• tested conda, pip, and poetry
• tested hocon, yaml
• tested docker images
• out of scope: env vars in yaml (as it's not capable of reading env vars, would have to change to another lib such as pyaml-env)
• minor fixes to the dockerfiles' command (just to make them work again, might need changes in other PRs)

[ChrsBaur]

@bertbesser if you find time please add a CHANGELOG.md entry and increase the version according to SemVer 2.0 :) I already did merge the current master into your branch.