Refactor image tags to be dynamic (#1120)

In order to simplify backporting and branching of new releases,
this patch relies on the Zuul branch when building images so
that we can easily branch out a new release and it will
automatically pick up the images.
diff --git a/Dockerfile b/Dockerfile
index e336f12..278b418 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -12,6 +12,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+ARG RELEASE
+
 FROM golang:1.21 AS go-builder
 COPY go.mod go.sum /src/
 WORKDIR /src
@@ -22,6 +24,6 @@
 COPY internal/ /src/internal/
 RUN go build -o main ./cmd/libvirt-tls-sidecar/main.go
 
-FROM registry.atmosphere.dev/library/ubuntu:main AS libvirt-tls-sidecar
+FROM registry.atmosphere.dev/library/ubuntu:${RELEASE} AS libvirt-tls-sidecar
 COPY --from=libvirt-tls-sidecar-builder /src/main /usr/bin/libvirt-tls-sidecar
 ENTRYPOINT ["/usr/bin/libvirt-tls-sidecar"]