ci: Run checks on master branch builds and all PRs

- This matches our current thinking in
  alphagov/govuk-rfcs#123, given that [push
  events don't run on forked branches in the forked repo, or when
  they're made into PRs](#337).

.github/workflows/tests.yml

@@ -1,6 +1,10 @@
 name: govuk-docker
 
-on: push
+on:
+  pull_request:
+  push:
+    branches:
+      - master
 
 jobs:
   test: