Add hostname flag to Docker docs

Fixes #4380

doc/020_installation.rst

@@ -265,6 +265,10 @@ binary, you can get it with `docker pull` like this:
 
     $ docker pull restic/restic
 
+Restic relies on the hostname for various operations. Make sure to set a static
+hostname using `--hostname` when creating a Docker container, otherwise Docker
+will assign a random hostname each time.
+
 From Source
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