support/testing: add docker / docker-compose tests

support/testing: add docker / docker-compose tests

Build for x86-64 as public containers in general are only available for
x86-64.  Docker needs a number of kernel options enabled, so use a custom
kernel config based on the qemu one.

Docker needs entropy at startup, so enable the virtio-rng-pci device to
expose entropy to the guest.  The default RAM amount (128M) is not enough to
run docker / docker-compose, so bump to 512MB.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>