Fix ordering of copy and go mod in Dockerfile

Once the api directory has its own go.mod, which is refered to in the
top go.mod, it needs to be copied before running `go mod download` in
the Dockerfile.

This won't have been picked up in PR testing, because it's only in the
Dockerfile and the docker-build isn't run in the test workflow (maybe
it should be?).

Signed-off-by: Michael Bridgen <michael@weave.works>

Dockerfile

@@ -7,12 +7,15 @@ WORKDIR /workspace
 COPY go.mod go.mod
 COPY go.sum go.sum
 
+# Copy this, which should not change often; and, needs to be in place
+# before `go mod download`.
+COPY api/ api/
+
 # cache modules
 RUN go mod download
 
 # copy source code
 COPY main.go main.go
-COPY api/ api/
 COPY controllers/ controllers/
 
 # build without giving the arch, so that it gets it from the machine