chore: try to reinforce CI by running integration tests in docker

.dockerignore

@@ -0,0 +1,2 @@
+_opam
+_build

.github/workflows/main.yml

@@ -29,16 +29,21 @@ jobs:
         allow-prerelease-opam: true
         dune-cache: true
           #- run: sudo apt install protobuf-compiler libprotobuf-dev
+    - run: bash dep/build-ci.sh
     - run: opam pin -n .
     - run: opam depext -yt ocaml-protoc
     - run: opam install -t . --deps-only
     - run: opam exec -- dune build @install
     - run: opam exec -- dune runtest
 
-    # integration tests
-    - run: opam install ppx_deriving -y
-    - run: sudo apt install protobuf-compiler libprotobuf-dev
-    - run: opam exec -- make integration PB_LINC=/usr/lib/x86_64-linux-gnu/ PB_HINC=/usr/include/x86_64-linux-gnu/
+  integration:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: true
+    - run: docker build -f dep/Dockerfile . --target=base
+    - run: docker build -f dep/Dockerfile . --target=run-the-damn-build
 
   format:
     name: format

dep/Dockerfile

@@ -0,0 +1,22 @@
+FROM ocaml/opam:debian-ocaml-4.14 AS base
+
+WORKDIR /build/
+RUN sudo chown -R opam: /build/
+RUN sudo apt install protobuf-compiler libprotobuf-dev -y
+COPY --chown=opam ./src/ ./src/
+COPY --chown=opam pbrt.opam pbrt_services.opam pbrt_yojson.opam ocaml-protoc.opam \
+  Makefile Makefile.test dune-project dune .ocamlformat \
+  .ocamlformat-ignore ./
+
+RUN opam pin -n .
+RUN opam depext -yt ocaml-protoc
+RUN opam install -t . --deps-only
+
+FROM base AS run-the-damn-build
+RUN opam exec -- dune build @install
+RUN opam exec -- dune runtest
+
+# integration tests
+RUN opam install ppx_deriving -y
+RUN opam exec -- make clean build integration PB_LINC=/usr/lib/x86_64-linux-gnu/ PB_HINC=/usr/include/x86_64-linux-gnu/
+