Add GH Actions workflow automatically building assets (#147)

.github/workflows/assets.yaml

@@ -0,0 +1,31 @@
+name: Assets build
+on:
+  push:
+    branches:
+      - main
+jobs:
+  main:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install Erlang & Elixir
+        uses: erlef/setup-elixir@v1
+        with:
+          otp-version: '23.2'
+          elixir-version: '1.11.3'
+      # Note: we need to get Phoenix and LV because package.json points to them directly
+      - name: Install mix dependencies
+        run: mix deps.get
+      - name: Install Node
+        uses: actions/setup-node@v2
+        with:
+          node-version: '14.x'
+      - name: Install npm dependencies
+        run: npm ci --prefix assets
+      - name: Build assets
+        run: npm run deploy --prefix assets
+      - name: Update assets
+        uses: stefanzweifel/git-auto-commit-action@v4
+        with:
+          commit_message: Update assets
+          file_pattern: priv/static