Docker support within the repository (#1732)

- Adds a Dockerfile within this repository
- GitHub actions to build and test the image

.github/workflows/docker-deploy.yml

@@ -0,0 +1,39 @@
+name: Deploy docker
+on: [push]
+env:
+  DEPLOY_NAME: gollumwiki/gollum:latest
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    if: ${{ github.ref == 'refs/heads/master' }}
+    steps:
+      - name: Check Out Repo
+        uses: actions/checkout@v2
+      - name: Login
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_HUB_USERNAME }}
+          password: ${{ secrets.DOCKER_HUB_ACCESS_TOKEN }}
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Cache docker layers
+        uses: actions/cache@v2
+        with:
+          path: /tmp/.buildx-cache
+          key: ${{ runner.os }}-buildx-${{ github.sha }}
+          restore-keys: |
+            ${{ runner.os }}-buildx-
+      - name: Build and push
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          context: ./
+          file: ./Dockerfile
+          builder: ${{ steps.buildx.outputs.name }}
+          push: true
+          tags: ${{ env.DEPLOY_NAME }}
+          cache-from: type=local,src=/tmp/.buildx-cache
+          cache-to: type=local,dest=/tmp/.buildx-cache
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}

.github/workflows/docker-test.yml

@@ -0,0 +1,38 @@
+name: Build and Test Docker
+on: [push, pull_request]
+env:
+  CI_IMAGE: gollum-ci-img
+jobs:
+  build-and-test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check Out Repo
+        uses: actions/checkout@v2
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Cache docker layers
+        uses: actions/cache@v2
+        with:
+          path: /tmp/.buildx-cache
+          key: ${{ runner.os }}-buildx-${{ github.sha }}
+          restore-keys: |
+            ${{ runner.os }}-buildx-
+      - name: Build
+        id: docker_build
+        uses: docker/build-push-action@v2
+        with:
+          context: ./
+          file: ./Dockerfile
+          builder: ${{ steps.buildx.outputs.name }}
+          push: false
+          tags: ${{ env.CI_IMAGE }}
+          outputs: type=docker
+          cache-from: type=local,src=/tmp/.buildx-cache
+          cache-to: type=local,dest=/tmp/.buildx-cache
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}
+      - name: docker state
+        run: docker image ls
+      - name: Run tests
+        run: docker run -e CI=true -w /app --entrypoint bundle ${{ env.CI_IMAGE }} exec rake