Investigate locale issues

[LukeCarrier]

Docker builds are emitting some nasty warnings:

[ERROR   ] Command 'grep "^LANG=" /etc/default/locale' failed with return code: 1
[ERROR   ] output: 

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
	LANGUAGE = (unset),
	LC_ALL = (unset),
	LANG = "en_GB.UTF-8 UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").

This is almost certainly related to the workaround we have to employ for the default PostgreSQL template, but I'm not entirely sure what's failing.

Moved from LukeCarrier/moodle-ubiquitous#19