Launch CI on push to main and pull requests (#21)

* Use on push to main and pull requests

* Use actions/checkout@v4

* Use root user as GitHub seems to support only root Docker users

.github/workflows/ci.yml

@@ -1,22 +1,23 @@
 name: CI
 
-on: [push]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
 
 jobs:
 
   test:
     runs-on: ubuntu-latest
     container:
       image: docker://firedrakeproject/firedrake-vanilla:latest
+      options: --user root
 
     steps:
-      # This Docker file changes sets USER to firedrake instead of using the default user, so we need to update file permissions for this image to work on GH Actions.
-      # See https://help.github.com/en/actions/reference/virtual-environments-for-github-hosted-runners#docker-container-filesystem
-      # (copied from https://help.github.com/en/actions/migrating-to-github-actions/migrating-from-circleci-to-github-actions)
-      - name: Setup file system permissions
-        run: sudo chmod -R 777 $GITHUB_WORKSPACE /github /__w/_temp
-
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 2
 
       - name: Python info
         shell: bash