Speed up incremental build

[tomwilkie]

• Pass -i to go build to speed up builds (forces go build to install dependancies, thanks @bboreham)
• Share scope/pkg into build container as /go/pkg (cached intermediate packages are installed here)
• Ignore files in ./vendor for Makefile dependancies
• Don't send two copies of the docker tarball to the docker daemon when building container
• Add make shell target to get a shell in the build container
• Don't install python-pip in the build container, just python-requests
• Update to go 1.5.2

[tomwilkie]

With this change, incremental builds take ~30s. Full build is 7mins.

$ make clean
$ make
...
real    6m54.132s
user    0m0.536s
sys     0m3.712s
$ # some very minor change
$ make
...
real    0m31.620s
user    0m0.124s
sys     0m2.352s

[tomwilkie]

For reference, on master incremental builds take almost 2mins:

$ make clean
$ make
$ # some very minor change
$ make
...
real    1m49.625s
user    0m0.996s
sys     0m5.480s

[paulbellamy]

If you can think of a better place to put the docker tarball that would be great, otherwise LGTM.