[temp] simplify how image name is sent to second dockerfile

Remove double :latest

.github/workflows/Dockerfile.copyin

@@ -1,7 +1,7 @@
 ARG builder
 
 # https://stackoverflow.com/a/63472135/3551604
-FROM ${TARGETPLATFORM}/${builder}:latest as builder
+FROM "${builder}" as builder
 
 FROM alpine:latest
 

.github/workflows/cross-compiling-docker.yml

@@ -22,6 +22,8 @@ jobs:
     - uses: actions/checkout@v2
     - name: set docker target
       run: echo "DOCKER_TARGET=$(./.github/workflows/get-compiler.sh docker)" >> $GITHUB_ENV 
+    - name: set docker image tag
+      run: echo "BUILDER_IMAGE_TAG=${DOCKER_TARGET}/${BUILDER_IMAGE_NAME}:latest" >> $GITHUB_ENV
     - uses: docker/setup-qemu-action@v1
     - uses: docker/setup-buildx-action@v1
       with:
@@ -32,10 +34,10 @@ jobs:
         context: .
         file: ./.github/workflows/Dockerfile.builder
         load: true # load into local image store
-        tags: "${{ env.DOCKER_TARGET }}/${{ env.BUILDER_IMAGE_NAME }}:latest"
+        tags: "${{ env.BUILDER_IMAGE_TAG }}"
         build-args: "GOARCH=${{ env.GOARCH }}"
     # - run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
-    - run: docker image inspect "${{ env.DOCKER_TARGET }}/${{ env.BUILDER_IMAGE_NAME }}:latest"
+    - run: docker image inspect "${{ env.BUILDER_IMAGE_TAG }}"
     - name: build monolith
       uses: docker/build-push-action@v2
       with:
@@ -44,4 +46,4 @@ jobs:
         platforms: ${{ env.DOCKER_TARGET }}
         push: true
         tags: localhost:5000/matrixdotorg/dendrite-monolith:latest
-        build-args: "builder=${{ env.BUILDER_IMAGE_NAME }}\nDENDRITE_BINARY=dendrite-monolith-server"
+        build-args: "builder=${{ env.BUILDER_IMAGE_TAG }}\nDENDRITE_BINARY=dendrite-monolith-server"