docker: small build time improvement
docker: small build time improvement

* Make a debug build for docker images
  (non-docker build is already debug build).
  qns test doesn't need a high performance so it should be ok
* No need to run `update-ca-certificates` (it will run at installation time)
* Add `git log..` for logging quiche commit is built. Only for
  travis log.
* No longer build with examples. Images will only have
  quiche-{client,server}

cloudflare/quiche,cloudflare/quiche,cloudflare/quiche