Reduce size of operator base image

- Uses the multi-stage pattern so the final stage is smaller (using the
  alpine variant) and copies in only what is needed from the
  dependencies stage.
- Reduces the image size from roughly 1.19GB to roughly 654MB.

Dockerfile.gitlab-operator-build-base

@@ -1,7 +1,4 @@
-FROM golang:1.14
-
-LABEL maintainer="GitLab Distribution Team" \
-      description="Build base image for the GitLab Operator project."
+FROM golang:1.14 AS dependencies
 
 ARG KUBEBUILDER_VERSION=2.3.1
 ARG YQ_VERSION=2.4.1
@@ -16,3 +13,14 @@ RUN go get -u \
 RUN curl --retry 6 -LsO https://github.com/mikefarah/yq/releases/download/${YQ_VERSION}/yq_linux_amd64 \
   && chmod +x yq_linux_amd64 \
   && mv yq_linux_amd64 /usr/local/bin/yq
+
+
+FROM golang:1.14-alpine
+
+LABEL maintainer="GitLab Distribution Team" \
+      description="Build base image for the GitLab Operator project."
+
+COPY --from=dependencies /go /go
+COPY --from=dependencies /usr/local/kubebuilder /usr/local/kubebuilder
+COPY --from=dependencies /usr/local/bin/ /usr/local/bin/
+COPY --from=dependencies /usr/local/bin/yq /usr/local/bin/yq