Default to vfs storage; configurable
Default to vfs storage; configurable

Make the storage configurable via the STORAGE_DRIVER
environment variable, defaulting to VFS.

The VFS option is particularly useful because it
works everywhere, all the time, including from
inside a 'docker build'.

The storage driver setting may be overwritten
by a Dockerfile's ENV statement, through the CLI,
or via the API.

Signed-off-by: Eric Windisch <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@windisch.us>

ewindisch/figleaf