Update Docker version in boot2docker ISO

[coolbrg]

Hi,

Currently, the boot2docker image version used to generate boot2docker.iso is 1.12.3.

However, Docker version is still lower than it i.e 1.11.2.

Upstream boot2docker repo also maintain the same version for both.
One definite reason would be "less confusion".

Unless we have any issues with docker version 1.12.3, we should update it.

Moreover, I found interesting facts today as:

#       Boot2docker version 1.12.3 boot2docker.iso 
Boot2Docker version 1.12.3, build HEAD : 7fc7575 - Thu Oct 27 17:23:17 UTC 2016
Docker version 1.12.3, build 6b644ec

#       Minishift master boot2docker.iso
Boot2Docker version 1.11.2, build HEAD : 7fc7575 - Thu Oct 27 17:23:17 UTC 2016
Docker version 1.11.2, build b9f10c9

Clearly, minishift master boot2docker.iso is showing wrong info of Boot2docker version for build HEAD 7fc7575.

[hferentschik]

Closing this issue here in favor of issue minishift/minishift-b2d-iso#6.