Use 'buildcache' for docker image build cache

Instead of re-purposing 'latest' tag, use 'buildcache' tag.
Since we deploy with linux/amd64, make that explicit as well.

The cache-from and cache-to which we now use with label/tag buildcache
as I understand it makes a docker build faster by avoiding pulling the
base image every time. In other words, I think GitHub's Container
Registry will have our previous image layers locally, and whenever the
first few layers are identical (majority case), it will be much faster
to re-use them.

The reason to use a separate tag for buildcache is that apparently it is
not a pullable/usable artifact with docker pull. The reason to keep the
latest tag is convenience for a one-off or exploratory pull.

Issue #39 no matching manifest for linux/amd64 on docker pull ...:latest

Author: bickelj

.github/workflows/build.yml

@@ -38,10 +38,11 @@ jobs:
         uses: docker/build-push-action@v3
         with:
           context: .
+          platforms: linux/amd64
           push: true
           tags: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ env.VERSION }}, ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest
-          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest
-          cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest,mode=max
+          cache-from: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache
+          cache-to: type=registry,ref=${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:buildcache,mode=max
       - name: Notify the top-level 'deploy' project that an image was pushed
         uses: aurelien-baudet/workflow-dispatch@v2.1.1
         with: