build: docker: Use host's network stack

build: docker: Use host's network stack

Armband build benefits (or should have) of a local mirror
for "linux.enea.com", which we used to inject into the build
system by host's "/etc/hosts".

After Armband switched to building inside docker, this change
was silently discarded by containerization.

The proposed solution uses hosts's network stack in all build
containers.

See [1] for full description.
This change simplifies the network abstraction during build by
NOT containerizing the containers' networking.

The original motivation was pointing to local mirrors via host's
"/etc/hosts", for which an alternative solution could be passing
"-v /etc/hosts:/etc/hosts".

[1] https://docs.docker.com/v1.8/articles/networking/

Change-Id: I31e3c787a432af49b4799f561669a0150668b222
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>