Deployment of custom rooms to Heroku

Description

The Heroku button is great for a very quick PoC that shows us briefly what #Matrix can do but now I wanna make deepers tests with it. I tried deploying manually (with Heroku CLI) by following the variables.env file instructions and I just can't make it work with Heroku deploys. Neither docker build nor heroku container:push allow us to use the env_file at build. I tried editing docker-entrypoint.sh file to source the env_file at run time but with no success whatsoever.

I see there is an issue with instructions to add variables manually in Heroku dashboard but we should really have a way to setup things with code.

Steps to Reproduce

First Try

1. Create variables.env file as per instructions.
2. edit docker-entrypoint.sh file to source variables.env at runtime.
3. Run heroku container:push web
4. Run heroku container:release web
5. If you run heroku logs --tail you'll see that the environment variables were not loaded. None of them.

Second Try

1. Instead of creating variables.env, edit heroku.yml and add variables to setup > config
2. Same results.

Expected Behavior

• Environment variables should be detected and setup should be properly executed.

Current Behavior

• No setup variables are detected. I can't create custom rooms neither setup Google App Id.

Suggestion

• Update documentation to allow us setup #Matrix in environments without Docker-Compose.