Allow setting slog handler from env var

[pcriv]

Simple PR to allow setting the slog Handler as a JSON handler by using the RIVER_LOG_FORMAT env variable

[brandur]

@bgentry Thoughts on this one? I initially thought this was on the main project and was a little skeptical about that, but we do use quite a few env vars in the UI already so it makes more sense though.

@pcriv I'm kind of thinking that if we go with this we should remove the abbreviation and use something like RIVER_LOG_FORMAT. "log fmt" sounds too much like "logfmt" [1], which is the informal name of the Heroku-style key/value logging pair format.

Ignore Docker build failures. Those builds don't work for third party contributors.

[1] https://brandur.org/logfmt

[pcriv]

Thanks for the suggestions! @brandur ofc I can update that 👍🏽

[bgentry]

I'm good with this generally, thanks for the suggestion! I have some specific ideas to clean it up, left comments on those.

Also could you please add yourself to the CLA?

[pcriv]

@bgentry I cleaned up the code a bit, lmk if it's better now

[bgentry]

Much better! 🤩 Thank you.

[pcriv]

Thanks to you folks! Happy to contribute just a little bit to this awesome project :)