Add env-file parameter to Docker Operator

[sshivprasad]

The change allows for passing a templateable .env file to Docker Operator to set the mentioned environment variables in the container. Passing more than a handful env variables might be cumbersome with environment, env-file would make it easier.

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[boring-cyborg]

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[potiuk]

I think it would be great to get it rebased if the tests fail.

[sshivprasad]

Thank you @pierrejeambrun and @eladkal

[boring-cyborg]

Awesome work, congrats on your first merged pull request!

[potiuk]

Cool addition to the Docker Operator. Thanks!

[sshivprasad]

Thank you for the guidance @potiuk !