docker/go/nethttp: go build -mod=mod

Go 1.16 changed the "go build" command so that
it no longer automatically updates go.mod and
go.sum. Because we dynamically change the Go
agent version, we cannot set go.mod and go.sum
ahead of time. Pass in `-mod=mod` to reinstate
the old behaviour.

docker/go/nethttp/Dockerfile

@@ -8,7 +8,7 @@ RUN git clone https://github.com/${GO_AGENT_REPO}.git /src/apm-agent-go
 RUN (cd /src/apm-agent-go \
   && git fetch -q origin '+refs/pull/*:refs/remotes/origin/pr/*' \
   && git checkout ${GO_AGENT_BRANCH})
-RUN CGO_ENABLED=0 go build
+RUN CGO_ENABLED=0 go build -mod=mod
 
 FROM alpine:latest
 RUN apk add --update curl && rm -rf /var/cache/apk/*