Docs: changelog for docker 6.3 (#18429)

packaging/docker/README.md

@@ -16,8 +16,11 @@ Further documentation can be found at http://docs.grafana.org/installation/docke
 
 ## Changelog
 
+### v6.3.0-beta2
+* Switch base image from ubuntu:latest to ubuntu:18.04
+
 ### v6.3.0-beta1
-* Docker: Switch base image to ubuntu:latest from debian:stretch to avoid security issues.
+* Switch base image to ubuntu:latest from debian:stretch to avoid security issues.
 
 ### v5.4.3
 * Build and publish docker images for armv7 and arm64 #14617, thx @johanneswuerbach