mingw: Bump docker base to Fedora 23.
mingw: Bump docker base to Fedora 23.

This build works as well. I'm not sure what's wrong with my
local environment.

xiph/opusfile,xiph/opusfile,xiph/opusfile,SmartWalkieOrg/opusfile,SmartWalkieOrg/opusfile,SmartWalkieOrg/opusfile