build: use docker for golang

As discussed in #1639.

Note that .dockerignore had to be changed as to *not* skip what's needed
for the build: vendor, obviously, and .git for the compiled-in version
information.

parts:
- .travis.yml: don't bother about golang anymore
- docs/devel/DEVELOPMENT.md: update
- Makefile: update hint

Signed-off-by: Stephan Renatus <srenatus@chef.io>