compose example: added explanation for not using named volumes

https://github.com/moby/moby/pull/17034#issuecomment-163361073

https://github.com/fphammerle/docker-reverse-encfs/commit/17c265231577434113812b89a26c9d7d0533234a
https://github.com/fphammerle/docker-reverse-encfs/commit/289dccb3424db8583cd903f5939bb916bf92297c

examples/rsync-sshd/docker-compose.yml

@@ -11,7 +11,14 @@ services:
       RECIPIENT: 1234567890ABCDEF1234567890ABCDEF12345678
     volumes:
     - plain_data:/plain:ro
-    # TODO replace host path with named volume
+    # Currently docker does not allow changing
+    # 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/rgpgfs:/encrypted:shared
     devices: [/dev/fuse]
     cap_add: [SYS_ADMIN]