Previously the `docker-compose.yml` we copied from GlitchTip's
instructions didn't specify a version tag. I don't know a ton about
Docker, but I imagine the deal here is that this means that we could
have used some Docker command like `docker pull` to update what version
of `glitchtip/glitchtip` is stored in our local image store or
something?
Anyway, I wanted to upgrade to version 4.0 (I'm not even actually sure
what version we were on before), and so the way I chose to do this was
to add a version specifier to the `docker-compose.yml` file. Then, all
we had to do was deploy it as usual, and Docker seemed to handle
downloading the image when starting up the service the next time, and
auto-performed the migrations, if I'm understanding correctly. (I did
see migrations being performed in the logs!) Great!
