Sets CGO_ENABLED to 0 in the Dockerfile.

scripts/consul-builder/Dockerfile

@@ -18,6 +18,9 @@ RUN mkdir /goroot && \
     curl https://storage.googleapis.com/golang/go${GOVERSION}.linux-amd64.tar.gz | \
          tar xzf - -C /goroot --strip-components=1
 
+# We want to ensure that release builds never have any cgo dependencies so we
+# switch that off at the highest level.
+ENV CGO_ENABLED 0
 ENV GOPATH /gopath
 ENV GOROOT /goroot
 ENV PATH $GOROOT/bin:$GOPATH/bin:$PATH