docs: clarify Docker image build availability (#4345)

docker-compose.yml

@@ -15,15 +15,17 @@
 #
 # If you wish to run with locally built KSQL docker images then build then:
 #
-# 1. ensure logged in to docker:
+# 1. Ensure logged in to docker:
 # > docker login
 #
-# 2. ensure you can log in to AWS ECR:
+# 2. Ensure you can log in to AWS ECR:
 # > aws ecr get-login --no-include-email | sh
 #
-# 3. build docker images from local changes (Note: access to Confluent docker registry is required):
-# Change `docker.upstream-tag` if you want to depend on anything other than the latest master upstream, e.g. 5.4.x-latest
-# > mvn -Pdocker package -DskipTests -Dspotbugs.skip -Dcheckstyle.skip  -Ddockerfile.skip=false -Dskip.docker.build=false -Ddocker.upstream-tag=master-latest -Ddocker.tag=local.build
+# 3. Build docker images from local changes 
+#    (NOTE: This currently depends on a Docker image that is not publicly available.
+#           Access to Confluent docker registry is required, and should be specified in docker.upstream-registry)
+#    Change `docker.upstream-tag` if you want to depend on anything other than the latest master upstream, e.g. 5.4.x-latest
+# > mvn -Pdocker package -DskipTests -Dspotbugs.skip -Dcheckstyle.skip  -Ddockerfile.skip=false -Dskip.docker.build=false -Ddocker.upstream-tag=master-latest -Ddocker.tag=local.build  -Ddocker.upstream-registry='XXXXXXXXX.dkr.ecr.us-west-2.amazonaws.com/'
 #
 # 4. check images build:
 # > docker image ls | grep ksql.local.build