Migrate to GitHub Actions (#100)

* Add actions workflow

* Update secrets for Docker push

.github/workflows/ci.yml

@@ -0,0 +1,60 @@
+name: Continuous Integration
+
+on:
+  push:
+    branches:
+      - '**'
+  pull_request:
+    branches:
+      - master
+  create:
+    tags:
+      - 'v[0-9]+.[0-9]+.[0-9]+*'
+
+env:
+  DOCKER_BUILDKIT: 1
+
+jobs:
+
+  build:
+    name: Build Artifacts
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v2
+      - name: Unit Tests
+        run: make test
+      - name: Build Prometheus-Exporter
+        run: make nginx-prometheus-exporter
+      - name: Build Image
+        uses: docker/build-push-action@v1
+        with:
+          repository: nginx/nginx-prometheus-exporter
+          dockerfile: build/Dockerfile
+          path: '.'
+          tag_with_sha: true
+          push: false
+
+  publish:
+    name: Publish Artifacts
+    runs-on: ubuntu-latest
+    needs: build
+    if: github.event_name == 'create' && contains(github.ref, 'refs/tags/')
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v2
+      - name: Retrieve Tag
+        id: get_version
+        run: echo ::set-output name=GIT_TAG::$(echo ${GITHUB_REF/refs\/tags\//} | tr -d v)
+      - name: Push Image
+        uses: docker/build-push-action@v1
+        with:
+          repository: nginx/nginx-prometheus-exporter
+          dockerfile: build/Dockerfile
+          path: '.'
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+          registry: ${{ secrets.DOCKER_REGISTRY }}
+          tag_with_ref: false
+          tags: latest, ${{ steps.get_version.outputs.GIT_TAG }}
+          push: true