Added "docker" workflow

.github/workflows/docker.yml

@@ -0,0 +1,40 @@
+name: docker
+
+on:
+  push:
+    branches:
+      - 'master'
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    
+    strategy:
+      matrix:
+        docker_path: [xenial, bionic, focal, bionic/android/rtabmap_api24, bionic/android/rtabmap_api26]
+        docker_tag: [xenial, bionic, focal, android24, android26]
+    
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v2
+      -
+        name: Set up QEMU
+        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: ${{ secrets.DOCKERHUB_TOKEN }}
+      -
+        name: Build and push
+        uses: docker/build-push-action@v2
+        with:
+          context: ./docker/${{ matrix.docker_path }}
+          push: true
+          tags: introlab3it/rtabmap:${{ matrix.docker_tag }}
+