Use BuildKit/buildx autopopulated ARGs for getting builder image

.github/workflows/Dockerfile.copyin

@@ -1,12 +1,16 @@
+ARG builder
+ARG TARGETARCH
+
+# https://stackoverflow.com/a/63472135/3551604
+FROM ${TARGETARCH}/${builder}:latest as builder
+
 FROM alpine:latest
 
-ARG BUILDER=
-ARG BUILDER_TAG=
-ARG DENDRITE_BINARY=
+ARG DENDRITE_BINARY
 
 ENV DENDRITE_BINARY=${DENDRITE_BINARY}
 
-COPY --from=${BUILDER}:${BUILDER_TAG} /build/bin/* /usr/bin
+COPY --from=builder /build/bin/* /usr/bin/
 
 VOLUME /etc/dendrite
 WORKDIR /etc/dendrite

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

@@ -20,6 +20,8 @@ jobs:
         - 5000:5000
     steps:
     - uses: actions/checkout@v2
+    - name: set docker target
+      run: echo "DOCKER_TARGET=$(./.github/workflows/get-compiler.sh docker)" >> $GITHUB_ENV 
     - uses: docker/setup-buildx-action@v1
       with:
         driver-opts: network=host
@@ -29,10 +31,9 @@ jobs:
         context: .
         file: ./.github/workflows/Dockerfile.builder
         load: true # load into local image store
-        tags: "${{ env.BUILDER_IMAGE_NAME }}:${{ matrix.target }}"
+        tags: "${{ env.DOCKER_TARGET }}/${{ env.BUILDER_IMAGE_NAME }}:latest"
         build-args: "GOARCH=${{ env.GOARCH }}"
     # - run: echo "RELEASE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
-    - run: echo "DOCKER_TARGET=$(./.github/workflows/get-compiler.sh docker)" >> $GITHUB_ENV 
     - uses: docker/setup-qemu-action@v1
     - name: build monolith
       uses: docker/build-push-action@v2
@@ -42,4 +43,4 @@ jobs:
         platforms: ${{ env.DOCKER_TARGET }}
         push: true
         tags: localhost:5000/matrixdotorg/dendrite-monolith:latest
-        build-args: "BUILDER=${{ env.BUILDER_IMAGE_NAME }}\nBUILDER_TAG=${{ env.GOARCH }}\nDENDRITE_BINARY=dendrite-monolith-server"
+        build-args: "builder=${{ env.BUILDER_IMAGE_NAME }}\nDENDRITE_BINARY=dendrite-monolith-server"