compose example: added docker/moby src refs for mount propagation restriction

examples/rsync-sshd-incl-gpg-enc-pwd/docker-compose.yml

@@ -15,6 +15,8 @@ services:
     # the mount propagation setting for named volumes.
     # https://github.com/moby/moby/pull/17034#issuecomment-163361073
     # https://github.com/moby/moby/pull/17034/files#diff-6896c3d2994ef80758bb7e38c07eb76bR103
+    # https://github.com/moby/moby/blob/e89b6e8c2d2c36c43f22aeaf2a885646c2994051/volume/linux_parser.go#L91
+    # https://github.com/moby/moby/blob/fc7b904dced4d18d49c8a6c47ae3f415d16d0c43/volume/validate.go#L74
     # https://github.com/moby/moby/blob/675144ff8d251a97322859a78f28ed4f988d3a74/volume/volume_unix.go#L100
     # So we bind mount a host dir instead.
     - /mnt/encrypted:/encrypted:shared