Dockerfile ADD caching bug

Dockerfile

FROM ubuntu

ADD foo.txt /foo.txt

foo.txt

old content

steps to repro

$ docker version
Client version: 0.7.4
Go version (client): go1.2
Git commit (client): 010d74e
Server version: 0.7.4
Git commit (server): 010d74e
Go version (server): go1.2
Last stable version: 0.7.4

$ docker build -t cache-test .
Uploading context 10.24 kB
Uploading context
Step 1 : FROM ubuntu
 ---> 8dbd9e392a96
Step 2 : ADD foo.txt /foo.txt
 ---> 1506c7614be0
Successfully built 1506c7614be0

$ docker run cache-test cat /foo.txt
old content

$ echo "new content" > foo.txt

$ docker build -t cache-test .
Uploading context 10.24 kB
Uploading context
Step 1 : FROM ubuntu
 ---> 8dbd9e392a96
Step 2 : ADD foo.txt /foo.txt
 ---> Using cache
 ---> 1506c7614be0
Successfully built 1506c7614be0

$ docker run cache-test cat /foo.txt
old content