resolve failure to update locale

[dragarcia]

The following error pops up in the recent Dockerbuild:

#8 3698.5 perl: warning: Setting locale failed.
#8 3698.5 perl: warning: Please check that your locale settings:
#8 3698.5 	LANGUAGE = (unset),
#8 3698.5 	LC_ALL = (unset),
#8 3698.5 	LANG = "en_US.utf8"
#8 3698.5     are supported and installed on your system.
#8 3698.5 perl: warning: Falling back to the standard locale ("C").

which then prevents the PG image from going up properly:

supabase-db        | bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8)
supabase-db        | The files belonging to this database system will be owned by user "postgres".
supabase-db        | This user must also own the server process.
supabase-db        | 
supabase-db        | initdb: error: invalid locale settings; check LANG and LC_* environment variables
supabase-db exited with code 1

Proposed fix is taken from this stack overflow question.