dapper v0.5.5 breaks rancher/os builds

[hpdvanwyk]

Steps to reproduce:

git clone https://github.com/rancher/os.git rancher-os
cd rancher-os
run make release

Output:

...snip...
+ tag=rancher/os-rootfs:628e6ab0
+ echo 'build-image: Building rancher/os-rootfs:628e6ab0'
build-image: Building rancher/os-rootfs:628e6ab0
+ '[' -x images/00-rootfs/prebuild.sh ']'
+ images/00-rootfs/prebuild.sh
+ dapper -d --build -f images/00-rootfs/Dockerfile -- -t rancher/os-rootfs images/00-rootfs
DEBU[0000] Building os:master using images/00-rootfs/Dockerfile
FATA[0000] pattern contains path separator
+ '[' 1 '!=' 42 ']'
+ exit 1
FATA[0353] exit status 1
make: *** [release-build] Error 1

It look like the following go commit is responsible: golang/go@12cd55c
I assume this was pulled in when updating the go version for v0.5.5.
Pinning the dapper version to v0.5.4 instead of latest in rancher/os allows it to build.