Build Dockerfile on CI (#6)

.github/workflows/ci.yml

@@ -7,13 +7,13 @@ on:
       - main
       - master
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
 
 jobs:
   ci:
     runs-on: ubuntu-latest
-    concurrency:
-      group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
-      cancel-in-progress: true
     steps:
       - uses: actions/checkout@v3
 
@@ -54,3 +54,24 @@ jobs:
       - name: Build the docs
         run: cargo doc --all-features --workspace --no-deps
         timeout-minutes: 15
+
+  build-docker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      # We've run into out-of-disk error when compiling Polkadot in the next step, so we free up some space this way.
+      - name: Free Disk Space (Ubuntu)
+        uses: jlumbroso/free-disk-space@54081f138730dfa15788a46383842cd2f914a1be # 1.3.1
+        with:
+          android: true # This alone is a 12 GB save.
+          # We disable the rest because it caused some problems. (they're enabled by default)
+          # The Android removal is enough.
+          dotnet: false
+          haskell: false
+          large-packages: false
+          swap-storage: false
+
+      - name: Build the Dockerfile
+        run: docker build . -t polkadot-sdk-minimal-template
+        timeout-minutes: 90