Speed up alpine build

[LaurentGoderre]

Fixes#693

[LaurentGoderre]

/cc @tianon

[LaurentGoderre]

BTW @SimenB@PeterDaveHello@chorrell the build errors. For some reasons the build output doesn't get to Travis. This could be caused by build being so verbose that the maximum Travis log length is reached.

Since we are using multi-stage, we could redirect the build output to file in the Dockerfile and if we need to access that log we can get if from the intermediary image.

What do you fellows think?

[chorrell]

I’m fine with reducing the verbosity of the logs a bit

[LaurentGoderre]

docker-library/official-images#3383 (comment)

[LaurentGoderre]

@chorrell maybe it's not the log length.....it's so weird. Is it possible that the docker process detaches?

[LaurentGoderre]

Something is weird with v9 alipine. It seems to fail often. I restarted just that one here and it's all green https://travis-ci.org/LaurentGoderre/docker-node/builds/371661398