docker: Add Hexagon image

docker: Add Hexagon image

This image is a little special because it takes a long time to build.
As such most users don't want to be doing that and just pull random
binaries from the ether as intended by the container gods. This
involves someone with credentials and a beefy machine running:

  make docker-image-debian-hexagon-cross V=1 NOCACHE=1 J=30
  docker tag qemu/debian-hexagon-cross registry.gitlab.com/qemu-project/qemu/qemu/debian-hexagon-cross
  docker push registry.gitlab.com/qemu-project/qemu/qemu/debian-hexagon-cross

With a suitable binary in the "cloud" a normal user will run:

  make docker-image-debian-hexagon-cross

or have it run for them through the dependency mechanism of our
over-engineered makefiles and get the binary they wanted. There are a
few wrinkles of course including needing to tweak the final image to
have the credentials of the user so we can actually do our cross
compiles.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Alessandro Di Federico <ale@rev.ng>
Cc: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Brian Cain <bcain@quicinc.com>
Message-Id: <20210512102051.12134-7-alex.bennee@linaro.org>