Accept start arguments through ENV variable

[aliismayilov]

In an environment like Github Actions it is not possible to modify the start command of services. So, the only way to run valkey with different server configuration would be to maintain a derived docker image where the default CMD is overwritten or custom valkey.conf is packaged.

It would be great to be able to pass start arguments to valkey-server through an environment variable and keep using the official image.

For reference, I like how ubuntu/redis image has it through REDIS_EXTRA_FLAGS env variable. I'd be happy to submit a pull request if maintainers agree to the suggestion.