Fixes#296
Today the kernel subdirectory is somewhat useless. The dockerfile we had
tried to use an image that isn't actually on ghcr, so everything would
kind of just crap out. This change swaps things so that we build a local
image that has all the bits needed to compile the kernel, and then uses
that image to build the resulting kernel and plop it in the current
working directory. This change does add a reliance on `container` being
installed however, but for folks that need to build a custom kernel this
doesn't seem like too much of an issue to me.
Michael Crosby