Added value format description for VOLUME instruction

In the documentation was not mentioned explicitly that VOLUME value
shoud be a valid JSON array. Because of this I spent time to discovering
the problem with my image where I put `VOLUME ['/data']` (with single quotes).
The `['/data']` mount point was parsed and mounted whole as a string without
any errors and warnings.
Docker-DCO-1.1-Signed-off-by: Max Shytikov <mshytikov@gmail.com> (github:
mshytikov)