Run unit tests without connecting to CI clusters

Summary

Uses envtest to remove the requirement for running tests against the live CI cluster.

Job times don't seem to have changed drastically:

Job name	Time on master	Time in this MR
slow_unit_tests[1]	8:31	8:19
slow_unit_tests[2]	8:04	8:09
unit_tests[1]	5:58	6:02
unit_tests[2]	6:24	5:40

Closes #221 (closed)

References

• envtest README.md
• envtest environment variables
• envtest server.go

Testing

Setting KUBECONFIG: "" in CI confirms that we don't have a connection to the CI cluster (#106 (closed) notes that KUBECONFIG needed to be exposed for tests to pass, and #177 (comment 638975999) notes that tests failed when we didn't expose KUBECONFIG).

To further validate, we can test locally in a container that is known to not have a connection to any live cluster. See instructions below.

1. Update Dockerfile

diff --git a/Dockerfile b/Dockerfile
index 9b74b10..7221b36 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -19,7 +19,7 @@ RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build -a -o manager
 
 # Use distroless as minimal base image to package the manager binary
 # Refer to https://github.com/GoogleContainerTools/distroless for more details
-FROM registry.access.redhat.com/ubi8/ubi-minimal:latest
+FROM registry.access.redhat.com/ubi8/ubi-minimal:latest AS release
 
 LABEL name=gitlab-operator \
       vendor='GitLab, Inc.' \
@@ -46,3 +46,32 @@ COPY --from=builder /workspace/manager .
 USER ${USER_UID}
 
 ENTRYPOINT ["/manager"]
+
+# ----- Stage: unit tests template -----
+
+FROM registry.gitlab.com/gitlab-org/gitlab-build-images:gitlab-operator-build-base AS unit_tests_template
+WORKDIR /workspace
+
+ENV HELM_CHARTS=/charts
+ENV CHART_VERSION=5.3.0
+ENV GITLAB_OPERATOR_ASSETS=/workspace/hack/assets
+ENV KUBEBUILDER_ASSETS=/usr/local/kubebuilder/bin
+ENV USE_EXISTING_CLUSTER="false"
+ENV KUBECONFIG=""
+
+COPY --from=release /charts /charts
+COPY . .
+
+# ----- Stage: unit tests -----
+
+FROM unit_tests_template AS unit_tests
+RUN mkdir coverage && /go/bin/ginkgo -skip 'controller' -cover -outputdir=coverage ./...
+
+# ----- Stage: slow unit tests -----
+
+FROM unit_tests_template AS slow_unit_tests
+RUN mkdir coverage && /go/bin/ginkgo -focus 'controller' -cover -outputdir=coverage ./...
+
+# ----- Stage: release (final) -----
+
+FROM release

2. Build image

$ ./scripts/retrieve_gitlab_charts.sh
$ mkdir -p .go/pkg/mod
$ docker build -t operator:local .

Ensure the build passes.