Change when Docker tests are run (#5)

## What
This limits when Docker tests are run:
- On pushes to `main`
- On pushes to branches that have a pull request associated to them, and
when said PR is opened

## Why
It's silly to run these tests on GitHub before I've opened a PR on
GitHub, because until I've done so I'm obviously not looking at GitHub
and I may push several times, which would mean running a bunch of
workflows I won't look at...

## Testing
None; this won't really have an effect until merged so I just hope I got
it right 😅

Edit: nah silly me, I can see that it was triggered when I opened this!
All good (just have to make sure the push to `main` also triggers it
tho)

.github/workflows/build-docker-image.yml

@@ -2,6 +2,9 @@ name: Build Docker image
 
 on:
   push:
+    branches: [main]
+
+  pull_request:
 
 jobs:
   docker: