docker-moby: add SRCREV_FORMAT

recipes that use multiple SCMs in the SRC_URI, must supply
SRCREV_FORMAT or SRCPV triggers an expansion error. While
this isn't fatal during the build, it can cause issues with
setscene (and possibly) other tasks failing, which then
leads to no sstate re-use, etc.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>