Define the ENV setting for all environments (including test/build/local)

[KevinMind]

Fixes: mozilla/addons#15358

Description

Defines a gloabl ENV setting and documents its purpose and meaning.

Testing

You can inspect for build,test,local.

Build

Anywhere in the dockerfile add

RUN echo "env: ${ENV}"

Run the build

make docker_build_web DOCKER_PROGRESS=plain

Expect to see "env: build"

Test

In any test add

print('env', os.environ.get('ENV'))
print('env_setting', settings.ENV)

Expect both to print "test"

Local

run make down && make up (to ensure new compose settings are applied

Check in the terminal

make shell
printenv

Expect ENV=local in there

Check in settings

make djshell
settings.ENV

Expect "local" to be returned

Screenshots

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.