fix docker pre-fetch of go modules

Signed-off-by: Morgan Tocker <tocker@gmail.com>

docker/bootstrap/Dockerfile.common

@@ -52,19 +52,21 @@ ENV USER vitess
 ENV GO111MODULE on
 
 # Copy files needed for bootstrap
-COPY bootstrap.sh dev.env build.env /vt/src/vitess.io/vitess/
+COPY bootstrap.sh dev.env build.env go.mod go.sum /vt/src/vitess.io/vitess/
 COPY config /vt/src/vitess.io/vitess/config
 COPY third_party /vt/src/vitess.io/vitess/third_party
 COPY tools /vt/src/vitess.io/vitess/tools
 COPY travis /vt/src/vitess.io/vitess/travis
 
-RUN go mod download
-
 # Create vitess user
 RUN groupadd -r vitess && useradd -r -g vitess vitess && \
     mkdir -p /vt/vtdataroot /home/vitess && \
     chown -R vitess:vitess /vt /home/vitess
 
+# Download vendored Go dependencies
+RUN cd /vt/src/vitess.io/vitess && \
+ su vitess -c "/usr/local/go/bin/go mod download"
+
 # Create mount point for actual data (e.g. MySQL data dir)
 VOLUME /vt/vtdataroot