docs: clarify docker build instruction (prometheus#16319)

Make it explicit that the make docker command is not intended for local builds.

Signed-off-by: Owen Williams <owen.williams@grafana.com>

Author: ywwg

README.md

@@ -130,7 +130,6 @@ always, be extra careful when loading third party code.
 
 ### Building the Docker image
 
-The `make docker` target is designed for use in our CI system.
 You can build a docker image locally with the following commands:
 
 ```bash
@@ -140,6 +139,9 @@ make npm_licenses
 make common-docker-amd64
 ```
 
+The `make docker` target is intended only for use in our CI system and will not
+produce a fully working image when run locally.
+
 ## Using Prometheus as a Go Library
 
 ### Remote Write