This will build an Ubuntu 14.04 image that is synced to the latest nexus 5 Boot2Gecko repositories (as of August 15, 2014).
You'll want to copy the system image off of your N5 to a
<backupsysdir>, and then run:
ANDROIDFS_DIR=<backupsysdir> ./config.sh nexus-5
within the /data/workspace/B2GN5 directory.
Docker Pull Command