ADD: Hint to function of docker version tags (#4967)

I wasn't sure about how tags work, maybe this is interesting for people with less knowledge of docker.

docs/content/doc/installation/with-docker.en-us.md

@@ -30,6 +30,8 @@ image as a service. Since there is no database available one can be initialized
 Create a directory like `gitea` and paste the following content into a file named `docker-compose.yml`.
 Note that the volume should be owned by the user/group with the UID/GID specified in the config file.
 If you don't give the volume correct permissions, the container may not start.
+Also be aware that the tag `:latest` will install the current development version.
+For a stable release you can use `:1` or specify a certain release like `:1.5.1`.
 
 ```yaml
 version: "2"