Dokku3: Update dokku to avoid stale build artefacts

[lucyb]

Why are we doing this?

It appears that there's an issue dokku/dokku#7216 with Dokku failing to clean up appropriately when deploying via git:from-image. This leads to stale build artefacts taking up space.

On Tuesday 12th Nov, Dokku4 ran out of disk space and it was then noticed that Dokku3 had used almost all of its disk space too. Running dokku cleanup didn't free up any additional space. However, running docker system prune did delete a large number of build cache objects and freed up around 70G of space. Taking disk usage down from (around) 93% to 56%. We think the update resolves this issue.

What are we doing?

Upgrading dokku to a version that includes the fix.

[lucyb]

Update: I've added a Digital Ocean alert for both dokku3 and dokku4, so tech support will be notified if we start running low on disk space. So this work is much less urgent now.

[mikerkelly]

@lucyb opensafely-core/opencodelists#2384 reduced the core + backup opencodelists disk requirements by about half (10s of GB), and opensafely-core/opencodelists#2382 covers docker system prune. So I'm closing this, please re-open if you disagree.

[lucyb]

@mikerkelly this issue appears to still be open, which is good because I think we should still do it. I have updated the original description, as it was missing the important point, which is that Dokku have released a fix that stop us from needing to do a periodic docker system prune.

[mikerkelly]

Thanks @lucyb I've re-named and re-written the description to foucs on updating dokku, and will remove the periodic prune from opensafely-core/opencodelists#2382