migrate to GitHub actions for builds

Author: joonas-fi

.github/workflows/build.yml

@@ -0,0 +1,18 @@
+name: Build
+
+on: [push]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Download Turbo Bob
+      run: curl --fail --location --output bob https://dl.bintray.com/function61/dl/turbobob/20200101_1536_caf624e6/bob_linux-amd64 && chmod +x bob
+    - name: Build with Turbo Bob
+      run: CI_REVISION_ID="$GITHUB_SHA" ./bob build --publish-artefacts
+      # unfortunately there doesn't seem to be a way to "expose all secrets", so you must
+      # list here each secret to pass on to the build
+      env:
+        DOCKER_CREDS: ${{ secrets.DOCKER_CREDS }}