all containers start from centos:centos6 instead of blalor/centos.
Official distro built images are considered more secure than unofficial
images built by unknown users shared on docker-hub

added a user 'bwa_user' to the containers
references data and reference volumes in /home/bwa_user/data and
/home/bwa_user/bwa_indexed
split_raw, bwa_aligner, and add_reference all run as USER bwa_user
to better secure the host machine

added .dockerignore to ignore the .git directory
Merge branch 'master' of https://github.com/dmlond/docker_bwa_aligner