Set up git workflow to build a maiko docker image with multi-architecture support

On pushes to master: builds images for x86_64, Apple M1, and ARMv7
architectures based on Ubuntu Focal release.  This builds only the
regular lde/ldex and does NOT build the ldeinit, so these are not
suitable for doing a loadup from scratch.

Commit based on https://github.com/stumbo/maiko/tree/gitHubBuild

.github/workflows/buildDocker.yml

@@ -0,0 +1,84 @@
+#  based on https://blog.oddbit.com/post/2020-09-25-building-multi-architecture-im/
+---
+# Interlisp workflow to build a Docker Image that supports multiple architectures
+name: 'Build Maiko Docker image'
+
+# Run this workflow on push to master
+# Other branches can be added it needed. 
+on:
+  push:
+    branches:
+      - master
+
+#  Jobs that are run as part of this workflow.
+jobs:
+  # Job to build the docker image
+  # see: https://github.com/docker/build-push-action
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      # Checkout the branch  
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      # Setup some environment variables
+      - name: Prepare
+        id: prep
+        run: |
+          # Name of the Docker Image. 
+          DOCKER_IMAGE=interlisp/${GITHUB_REPOSITORY#*/}
+          VERSION=latest
+          SHORTREF=${GITHUB_SHA::8}
+          ## Do we want to use tags and or versions 
+          # If this is git tag, use the tag name as a docker tag
+          if [[ $GITHUB_REF == refs/tags/* ]]; then
+            VERSION=${GITHUB_REF#refs/tags/v}
+          fi
+          TAGS="${DOCKER_IMAGE}:${VERSION},${DOCKER_IMAGE}:${SHORTREF}"
+          # If the VERSION looks like a version number, assume that
+          # this is the most recent version of the image and also
+          # tag it 'latest'.
+          if [[ $VERSION =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
+            TAGS="$TAGS,${DOCKER_IMAGE}:latest"
+          fi
+          # Set output parameters.
+          echo ::set-output name=tags::${TAGS}
+          echo ::set-output name=docker_image::${DOCKER_IMAGE}
+          echo ::set-output name=build_time::$(date -u +'%Y-%m-%dT%H:%M:%SZ')
+      # Setup the Docker Machine Emulation environment.  
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@master
+        with:
+          platforms: all
+
+      # Setup the Docker Buildx funtion
+      - name: Set up Docker Buildx
+        id: buildx
+        uses: docker/setup-buildx-action@master
+
+      # Login into DockerHub - required to store the created image
+      - name: Login to DockerHub
+        if: github.event_name != 'pull_request'
+        uses: docker/login-action@v1
+        with:
+          username: ${{ secrets.DOCKER_USERNAME }}
+          password: ${{ secrets.DOCKER_PASSWORD }}
+
+      # Start the Docker Build using the Dockerfile in the repository we
+      # checked out.  
+      - name: Build
+        uses: docker/build-push-action@v2
+        with:
+          builder: ${{ steps.buildx.outputs.name }}
+          build-args: BUILD_DATE=${{ steps.prep.outputs.build_time }}
+          context: .
+          file: ./Dockerfile
+          # Platforms - Sepecify the platforms to include in the build
+          #  linux/amd64 -- Standard x86_64
+          #  linux/arm64 -- Apple M1
+          #  linux/arm/v7 -- Raspberry pi
+          platforms: linux/amd64,linux/arm64,linux/arm/v7
+          # Push the result to DockerHub
+          push: true
+          # tags to assign to the Docker image
+          tags: ${{ steps.prep.outputs.tags }}