Fix shell escaping of redis/memcached password variables

[timabbott]

A user reported seeing this output when running docker-compose operations for the Zulip container:

~/docker-zulip$ sudo docker-compose pull
[sudo] Passwort für ilyas:
WARNING: The GVqF variable is not set. Defaulting to a blank string.
Pulling database  ... done
Pulling memcached ... done
Pulling rabbitmq  ... done
Pulling redis     ... done

(Note that the zulip container isn't being run at all).

Investigating determined the problem was that their REDIS_PASSWORD field had some special characters in it (in this case, $), which resulted in Docker trying to handle it as a variable.

REDIS_PASSWORD: 'E!r%ZwH3#2P$GVqF'

See https://pastebin.com/P0jRFVnj for an example configuration with this problem.

I'm not sure what the right fix for this; at the very least we should document that passwords/secrets used in this file should use a specific character set; I'm not sure there's any escaping we can do to address this, since I think it's being considered a Docker variable. May be worth researching what the prior art for other projects is for this issue. One possible solution is to write a tool to generate and fill in the password/secrets fields rather than having folks do that manually.

[andersk]

$VARIABLE substitution is part of the docker-compose.yml syntax, which also provides a syntax for escaping $:

You can use a $$ (double-dollar sign) when your configuration needs a literal dollar sign. This also prevents Compose from interpolating a value, so a $$ allows you to refer to environment variables that you don’t want processed by Compose.

web:
  build: .
  command: "$$VAR_NOT_INTERPOLATED_BY_COMPOSE"

If you forget and use a single dollar sign ($), Compose interprets the value as an environment variable and warns you:

The VAR_NOT_INTERPOLATED_BY_COMPOSE is not set. Substituting an empty string.

[timabbott]

Yeah. I think we should address this by updating our docs to mention not using $ in passwords; or maybe better, we should write a little tool that substitutes in randomly generated passwords following that rule.

[i-ky]

Users can be advised to use docker compose config to verify that Docker Compose interprets their Compose file as intended.