Make DOCKER_TARGET a buid time argument only.

[KevinMind]

Fixes: mozilla/addons#15361

Description

Define TARGET setting reading the docker target from build info.

Context

It is very subtle, but extremely problematic to rely on an environmnet variable for DOCKER_TARGET. This value is mostly relevant during build time, it literally determines which stage of the docker image to build to. The actual state cannot be changed, but environment variables can be freely changed whenever you want.

Since we are relying on this value to know what kind of checks to run or what features might be enabled we must use a value that is hard coded during the build. that is what the build-info.json is meant for.

Testing

Make up with a given target "production" or "development"

make up DOCKER_TARGET=<target>

Shell into the container

make shell

Print the environmnet

Expect DOCKER_TARGET is not defined

printenv

Django shell it up

make djshell

Get that TARGET setting

settings.TARGET

Expect to match <target>

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.