Add docker build stage on Github actions

.github/workflows/build.yml

@@ -117,6 +117,30 @@ jobs:
           valgrind --leak-check=full --leak-check-heuristics=all --undef-value-errors=no --error-exitcode=9 ./bin/minetest --run-unittests
 
 
+  clang_9_prometheus:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install compiler
+        run: |
+          sudo apt-get install clang-9 -qyy
+          source ./util/ci/common.sh
+          install_linux_deps
+
+      - name: Build prometheus-cpp
+        run: |
+          ./util/ci/build_prometheus_cpp.sh
+
+      - name: Build
+        run: |
+          ./util/ci/build.sh
+        env:
+          CMAKE_FLAGS: "-DCMAKE_C_COMPILER=clang-9 -DCMAKE_CXX_COMPILER=clang++-9 -DENABLE_PROMETHEUS=1"
+
+      - name: Test
+        run: |
+          ./bin/minetest --run-unittests
+
   # Some builds doesn't require freetype, ensure it compiled properly
   clang_9_no_freetype:
     runs-on: ubuntu-18.04
@@ -138,6 +162,14 @@ jobs:
         run: |
           ./bin/minetest --run-unittests
 
+  docker:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Build docker image
+        run: |
+          docker build .
+
   win32:
     runs-on: ubuntu-18.04
     steps: