Trac #33263: Add github action running on each push

Add a github action that runs on each push. Since it uses the most
recent docker image as the base, the runtime is relatively short with
about 2h. The idea is to get quick feedback on PRs without the need to
wait until the patchbot picks it up. After this ticket is merged, we can
add a badge in the ticket similar to the linter workflow.

Example run: https://github.com/sagemath/sagetrac-
mirror/actions/workflows/build.yml
(it works but one doctest and the pytests are failing; I think there are
already tickets for these issues)

URL: https://trac.sagemath.org/33263
Reported by: gh-tobiasdiez
Ticket author(s): Tobias Diez
Reviewer(s): Matthias Koeppe

.github/workflows/build.yml

@@ -0,0 +1,34 @@
+name: Build & Test
+
+on:
+  push:
+  workflow_dispatch:
+    # Allow to run manually
+
+concurrency:
+  # Cancel previous runs of this workflow for the same branch
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container: ghcr.io/sagemath/sage/sage-docker-ubuntu-focal-standard-with-targets:dev
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Prepare
+        run: |
+          # Reuse built SAGE_LOCAL contained in the Docker image 
+          ./bootstrap
+          ./configure --enable-build-as-root --prefix=/sage/local --with-sage-venv
+      
+      - name: Build
+        run: make build
+        env:
+          MAKE: make -j2
+          SAGE_NUM_THREADS: 2
+
+      - name: Test
+        run: ./sage -t --all -p0