WIP: read config from environment variables

[vichle]

This is a first draft of reading configuration from environment variables (#561) . I chose a different approach than #1041, let me know what you think! I've only added support for a single arg for now: backend using the FAKE_GCS_BACKEND environment variable. I'll update the PR with the rest of the arguments if you think this is a good way of doing it.

I don't usually write golang, so please be thorough in your feedback :)

Open questions:

• Is this a good approach?
• Should all arguments be supported?
• Do we need more tests?

[vichle]

@fsouza some feedback would be appreciated :)

[fsouza]

@fsouza some feedback would be appreciated :)

Sorry I completely missed this, will take a look later today!

[Isakdl]

@fsouza any love for this? :) Would be great to have support for envs for github actions services.

[fsouza]

Apologies for such a bad turnaround.

I think this is fine as it behaves as expected: flag has precedence, but environment variable can be used to override the default value.

If you want to handle all args that'd be appreciated. We need a solution for flags that are not strings, but we don't need to make it too generic given we only have to handle strings and uint.

I'm comfortable with the basic testing you have and maybe exhaustive testing for envVarOrDefault.

Thanks for contributing!