docs: link to Docker image, APT, in release notes

relnotes/v2.md

@@ -0,0 +1,6 @@
+This release is also available as:
+
+* APT repository: https://apt.syncthing.net/
+
+* Docker image: `docker.io/syncthing/syncthing:{{.version}}` or `ghcr.io/syncthing/syncthing:{{.version}}`
+  (`{docker,ghcr}.io/syncthing/syncthing:2` to follow just the major version)