Use docker system prune

[sau226]

This commit updates the repository to use Docker's built in method of
cleaning up old containers per Sam Saffron's request in #416.

The cleanup command and 1 instance where docker system prune was already used will call docker system prune -f (don't prompt the user for a yes/no to trigger the command) as the launcher script will prompt the user for confirmation before running docker system prune.

P.S: If you are working for Discourse and have write access to this repo feel free to add the -a (remove stopped containers as well as all unused images) option to where I've used docker system prune. I want your opinon on this so if you want/need it then feel free to commit that to my fork before merging (I'm fine with that).

[SamSaffron]

Thanks heaps for the feedback here, I went ahead with the change (see last 2 commits) I strangely forgot to push my first attempt at it yesterday.

Let me know what you think.

[sau226]

@SamSaffron I'd suggest using restoring the old code (in ./launcher only) and restoring the code from your first commit (but replace STATE_DIR=./.gc-state scripts/docker-gc with $docker_path system prune -a -f or similar).

Discourse also used to purge old logs by running rm -f shared/standalone/log/var-log/*.txt after the gc script would otherwise run. It doesn't appear to run any more now. This proposed restoration would allow the gate to be able to prune the containers and delete all the logs in 1 go.

If needed I can file a PR for this or an issue on meta. Let me know what you think.

[SamSaffron]

not sure about this *.txt in var logs is a relic of the past, no need to worry about cleaning it up anymore.

if we can figure out extra stuff to remove on cleanup I am open to that ... but I fine the text we get back from prune -a to be fine for now.