ARANGODB_STARTER_MODE and similar env vars are ignored

[ex-nerd]

I've been trying to run a container inside of github-actions with starter.mode=single but it turns out the entrypoint script for this container doesn't actually support the full set of startup environment variables.

I'm building a standalone action as a workaround with a wrapper entrypoint script, but it would be nice for a variety of reasons to be able to specify start-mode and other similar flags via environment variables instead of having to wrap entrypoint (not the least of which is the fact that this container doesn't match up with the arangodb documentation, but also for simpler docker-compose setups)