Docker version 1.11.2, build b9f10c9
OSX 10.10.5 (14F1808)
Boot2Docker version 1.11.2, build HEAD : a6645c3 - Wed Jun 1 22:59:51 UTC 2016

First create a file in your home directory with the content test AAAA
localmachine$ printf "test AAAA" > ~/cache.test

Next log into the boot2docker image
localmachine$ docker-machine ssh

Verify the file is readable. (note i used "*" because i didn't want to post my username. You will get the same results either way)

docker@default:~$ cat /Users/*/cache.test
test AAAAdocker@default:~$

Now open the file and modify the contents to test BBBB

docker@default:~$ vi /Users/*/cache.test

and change the line to

test BBBB

Now cat the file

docker@default:~$ cat /Users/*/cache.test
test AAAAdocker@default:~$

Need to remount to fix the issue

docker@default:~$ sudo mount -o remount,rw /Users
docker@default:~$ cat /Users/*/cache.test
test BBBB

Whats going on here?!