Two consecutive runs of `docker build .` use different caches!

build output

[gurjeet@work:8 ~/dev/xDBAutoTests] T231827 (master)
$ docker build -rm  .
Uploading context 51200 bytes
Step 1 : FROM ubuntu
 ---> 8dbd9e392a96
Step 2 : RUN apt-get update
 ---> Using cache
 ---> 1b917ec69361
Step 3 : RUN apt-get install -y lsb-release
 ---> Using cache
 ---> 9d9b1d71e67e
Step 4 : RUN apt-get install -y python-software-properties
 ---> Using cache
 ---> 22df072d209f
Step 5 : RUN add-apt-repository "deb http://archive.ubuntu.com/ubuntu precise universe"
 ---> Using cache
 ---> 385ae4bf6980
Successfully built 385ae4bf6980
[gurjeet@work:8 ~/dev/xDBAutoTests] T231828 (master)
$ docker build -rm  .
Uploading context 51200 bytes
Step 1 : FROM ubuntu
 ---> 8dbd9e392a96
Step 2 : RUN apt-get update
 ---> Using cache
 ---> 1b917ec69361
Step 3 : RUN apt-get install -y lsb-release
 ---> Using cache
 ---> 9d9b1d71e67e
Step 4 : RUN apt-get install -y python-software-properties
 ---> Using cache
 ---> e77ee88d4280
Step 5 : RUN add-apt-repository "deb http://archive.ubuntu.com/ubuntu precise universe"
 ---> Running in 10ae99b2649c
/bin/sh: 1: add-apt-repository: not found
Error build: The command [/bin/sh -c add-apt-repository "deb http://archive.ubuntu.com/ubuntu precise universe"] returned a non-zero code: 127
The command [/bin/sh -c add-apt-repository "deb http://archive.ubuntu.com/ubuntu precise universe"] returned a non-zero code: 127

docker version

[gurjeet@work:1 ~/dev/xDBAutoTests] T230551 (master)
$ docker version
Client version: 0.6.3
Go version (client): go1.1.2
Git commit (client): b0a49a3
Server version: 0.6.3
Git commit (server): b0a49a3
Go version (server): go1.1.2
Last stable version: 0.6.3

Dockerfile

FROM ubuntu

# Update the software repositories
RUN apt-get update

# Install package for lsb_release binary
RUN apt-get install -y lsb-release

# Install package  that provides add-apt-repository
RUN apt-get install -y python-software-properties

# Add the 'universe' repository to package sources list
RUN add-apt-repository "deb http://archive.ubuntu.com/ubuntu precise universe"

Third Run Output

[gurjeet@work:8 ~/dev/xDBAutoTests] T231857 (master)
$ docker build -rm  .
Uploading context 51200 bytes
Step 1 : FROM ubuntu
 ---> 8dbd9e392a96
Step 2 : RUN apt-get update
 ---> Using cache
 ---> cd4713919099
Step 3 : RUN apt-get install -y lsb-release
 ---> Using cache
 ---> 4f00d63c5a56
Step 4 : RUN apt-get install -y python-software-properties
 ---> Using cache
 ---> 0a72ad35ec38
Step 5 : RUN add-apt-repository "deb http://archive.ubuntu.com/ubuntu precise universe"
 ---> Using cache
 ---> abe61d110082
Successfully built abe61d110082