cli: Install first config before running docker pull

In some very niche corner cases (devs wanting to run fastpath on a
system that initially has a very custom kernel installed), the initial
running kernel does not have sufficient Kconfigs enabled to run docker
(this is true for defconfig for example). Therefore "docker image pull"
would fail. Let's workaround this by installing the first config first
then we can do the "docker image pull" in the context of that kernel
which must support docker.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>