isolate docker build source in src/ subdir
add build script for convenience
add bootstrap script for convenience
reduce size of built image by about 30 MiB

* move docker build steps into a bash script for single fs layer
* remove dev tools after building authproxy to improve security
reduce image size to 289 MiB

Split the build into 3 stages:

1. common image provides centos6 and "duo" user.
2. builder image builds the duoauthproxy software.
3. runtime image provides just the runtime software without dev tools.
runtime container starts duoauthproxy directly by default
add workflow for contributions

Note: This is from https://github.com/coreos/etcd-ca/ with minor tweaks
because its contributor workflow aligns with my own inclinations.