Share the Go build cache when building in Dockerfiles (#1853)

On my machine this makes image rebuilds go from ~5m to 1.5s.

This also required setting DOCKER_BUILDKIT=1 in integration test image
builds.

I also took the opportunity to bump the Go version used in those tests
to Go 1.17 to match the image build processes, and tidied up the
workflow files a bit too (renaming for consistency, typos, whitespace,
etc.)

deploy/Dockerfile_debug

@@ -35,8 +35,11 @@ RUN go install github.com/chrismellard/docker-credential-acr-env@09e2b5a8ac86c3e
 RUN mkdir -p /kaniko/.docker
 
 COPY . .
-RUN make GOARCH=$TARGETARCH
-RUN make GOARCH=$TARGETARCH out/warmer
+RUN \
+    --mount=type=cache,target=/root/.cache/go-build \
+    --mount=type=cache,target=/go/pkg \
+    make GOARCH=$TARGETARCH && \
+    make GOARCH=$TARGETARCH out/warmer
 
 # Generate latest ca-certificates