Add GitHub workflow to build Docker image from tags

This workflow will be triggered when any tag, that match the semi-
regex (it is not normal regex, since "." is treated as a normal
period here), is pushed to the repository. The helper script will
then properly extract the version numbers we are interested in,
before the Docker image is built and tagged properly.

More info about this can be found in issue #28:
#28

.github/version_extractor.sh

@@ -0,0 +1,21 @@
+# This is a small helper script used to extract and verify the tag that is to
+# be set on the Docker container. This file expects that the GitHub Action
+# variable GITHUB_REF is passed in as the one and only argument.
+
+if [ -z "${1}" ]; then
+    echo "Input argument was empty"
+    exit 1
+fi
+
+app_version=$(echo ${1} | sed -n -r -e 's&^refs/.+/v([1-9]\.[0-9]+\.[0-9]+).*$&\1&p')
+nginx_version=$(echo ${1} | sed -n -r -e 's&^refs/.+/.*-nginx([1-9]\.[0-9]+\.[0-9]+)$&\1&p')
+
+if [ -n "${app_version}" -a -n "${nginx_version}" ]; then
+    echo "::set-output name=APP_VERSION::${app_version}"
+    echo "::set-output name=NGINX_VERSION::${nginx_version}"
+else
+    echo "Could not extract all expected values:"
+    echo "APP_VERSION=${app_version}"
+    echo "NGINX_VERSION=${nginx_version}"
+    exit 1
+fi

.github/workflows/build_tags.yml

@@ -0,0 +1,48 @@
+name: "build-tags"
+
+on:
+  push:
+    branches-ignore:
+      - "**"
+    tags:
+      - "v[1-9].[0-9]+.[0-9]+-nginx[1-9].[0-9]+.[0-9]+"
+
+jobs:
+  docker_buildx:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@v2
+
+      - name: Extract version numbers from GitHub reference
+        id: tagger
+        run: bash .github/version_extractor.sh ${GITHUB_REF}
+
+      - name: Set up QEMU environment
+        uses: docker/setup-qemu-action@v1
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Login to DockerHub
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Build and push all images
+        uses: docker/build-push-action@v2
+        with:
+          context: ./src
+          platforms: |
+            linux/amd64
+            linux/386
+            linux/arm64
+            linux/arm/v7
+          pull: true
+          no-cache: true
+          push: true
+          build-args: BUILDX_QEMU_ENV=true
+          tags: |
+            jonasal/nginx-certbot:${{ steps.tagger.outputs.APP_VERSION }}
+            jonasal/nginx-certbot:${{ steps.tagger.outputs.APP_VERSION }}-nginx${{ steps.tagger.outputs.NGINX_VERSION }}