Add optional $operator_version Dockerfile arg.

Value is passed to `go build` process and dynamically updates the version.Version variable.

Dockerfile

@@ -12,7 +12,15 @@ COPY pkg/ pkg/
 COPY version/ version/
 COPY vendor/ vendor/
 # Build
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build -mod vendor -a -o manager main.go
+ARG operator_version=dev
+RUN CGO_ENABLED=0 \
+    GOOS=linux \
+    GOARCH=amd64 \
+    GO111MODULE=on \
+    go build \
+    -ldflags "-X version.Version=$operator_version" \
+    -mod vendor \
+    -a -o manager main.go
 
 # Use distroless as minimal base image to package the manager binary
 # Refer to https://github.com/GoogleContainerTools/distroless for more details