PMM-14568 Add a special label for watchtower (#4766)

Author: ademidoff

build/docker/server/Dockerfile.el9

@@ -38,6 +38,7 @@ LABEL org.opencontainers.image.licenses=AGPL-3.0
 LABEL org.opencontainers.image.title="Percona Monitoring and Management"
 LABEL org.opencontainers.image.vendor="Percona LLC"
 LABEL org.opencontainers.image.version=${VERSION}
+LABEL com.percona.pmm=true
 
 USER 1000