Add GitHub Action to build image on push

.github/workflows/on-push.yml

@@ -0,0 +1,59 @@
+name: Docker build on push
+
+env:
+    DOCKER_CLI_EXPERIMENTAL: enabled
+
+on:
+  push:
+    branches:
+      - master
+      - docker-image # For testing purposes
+
+jobs:
+    build:
+        runs-on: ubuntu-20.04
+        name: Build and push lnbits image
+        steps:
+            - name: Login to Docker Hub
+              run: echo "${{ secrets.DOCKER_PASSWORD }}" | docker login -u "${{ secrets.DOCKER_USERNAME }}" --password-stdin
+
+            - name: Checkout project
+              uses: actions/checkout@v2
+
+            - name: Set up QEMU
+              uses: docker/setup-qemu-action@v1
+              id: qemu
+
+            - name: Setup Docker buildx action
+              uses: docker/setup-buildx-action@v1
+              id: buildx
+
+            - name: Show available Docker buildx platforms
+              run: echo ${{ steps.buildx.outputs.platforms }}
+
+            - name: Cache Docker layers
+              uses: actions/cache@v2
+              id: cache
+              with:
+                path: /tmp/.buildx-cache
+                key: ${{ runner.os }}-buildx-${{ github.sha }}
+                restore-keys: |
+                  ${{ runner.os }}-buildx-
+
+            - name: Run Docker buildx against commit hash
+              run: |
+                  docker buildx build \
+                  --cache-from "type=local,src=/tmp/.buildx-cache" \
+                  --cache-to "type=local,dest=/tmp/.buildx-cache" \
+                  --platform linux/amd64,linux/arm64,linux/arm/v7 \
+                  --tag ${{ secrets.DOCKER_USERNAME }}/lnbits:${GITHUB_SHA:0:7} \
+                  --output "type=registry" ./
+
+            - name: Run Docker buildx against latest
+              run: |
+                  docker buildx build \
+                  --cache-from "type=local,src=/tmp/.buildx-cache" \
+                  --cache-to "type=local,dest=/tmp/.buildx-cache" \
+                  --platform linux/amd64,linux/arm64,linux/arm/v7 \
+                  --tag ${{ secrets.DOCKER_USERNAME }}/lnbits:latest \
+                  --output "type=registry" ./