Cleanup .env{,.custom} handling

[chadwhitacre]

Fixes #941, also addresses the suggestion for a README tweak at the very bottom of #1491 (comment).

Look good to you, @aminvakil @glensc @Daniel15?

[Daniel15]

This looks good to me!

[aminvakil]

I tested this locally on a clean machine (running ./integration-test.sh) and faced this error:

./_integration-test/../install/_lib.sh: line 28: /home/fedora/self-hosted/_integration-test/.env: No such file or directory

[chadwhitacre]

What's up with CI? 🤔

  time="2022-06-23T00:09:18Z" level=warning msg="The \"HEALTHCHECK_INTERVAL\" variable is not set. Defaulting to a blank string."
  time="2022-06-23T00:09:18Z" level=warning msg="The \"HEALTHCHECK_TIMEOUT\" variable is not set. Defaulting to a blank string."
  time="2022-06-23T00:09:18Z" level=warning msg="The \"HEALTHCHECK_RETRIES\" variable is not set. Defaulting to a blank string."
  time="2022-06-23T00:09:18Z" level=warning msg="The \"SNUBA_IMAGE\" variable is not set. Defaulting to a blank string."
  time="2022-06-23T00:09:18Z" level=warning msg="The \"SENTRY_BIND\" variable is not set. Defaulting to a blank string."
  time="2022-06-23T00:09:18Z" level=warning msg="The \"HEALTHCHECK_TIMEOUT\" variable is not set. Defaulting to a blank string."
  time="2022-06-23T00:09:18Z" level=warning msg="The \"HEALTHCHECK_RETRIES\" variable is not set. Defaulting to a blank string."
  error while interpolating services.redis.healthcheck.retries: failed to cast to expected type: strconv.Atoi: parsing "": invalid syntax

https://github.com/getsentry/self-hosted/runs/7014638001?check_suite_focus=true#step:4:1313

This isn't a familiar CI flake. Is it authentic?

[chadwhitacre]

Is it authentic?

Yup, and it's downstream of @aminvakil's result:

./_integration-test/../install/_lib.sh: line 28: /home/runner/work/self-hosted/self-hosted/_integration-test/.env: No such file or directory

https://github.com/getsentry/self-hosted/runs/7096779775?check_suite_focus=true#step:4:1258

[emmatyping]

This looks good!

Its a shame there isn't a shell linter that reports macOS/linux incompatibilities :/

[chadwhitacre]

a shell linter that reports macOS/linux incompatibilities

Oooooh ... good idea! :)