We have to use the final location of IGT for --prefix as some of the
binaries use that for asset lookup (e.g. register definitions that end
up in $PREFIX/share/).

To install it under a local directory during the early build steps it's
better to use $DESTDIR.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>

We were pulling and tagging images locally ahead of time just in case
the next build may need it (i.e. debian-minimal for the fully featured
debian).

Instead of doing that we can specify the registry in Dockerfile's FROM
cluse, so the image is pulled only when we are need it.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>

Correct one = the one tagged with current commit, so in case of parallel
pipelines we don't take the wrong "latest".

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>

Let's install all the necessary things in the build-* images, so we
don't touch/hammer external servers each time there is a small change in
IGT.

We have already seen this failing:

	Error: Error downloading packages:
	  Status code: 503 for https://mirrors.fedoraproject.org/metalink?repo=fedora-30&arch=x86_64


	The command '/bin/sh -c dnf install -y findutils &&         dnf clean all' returned a non-zero code: 1

Cc: Oleg Vasilev <oleg.vasilev@intel.com>
Cc: Petri Latvala <petri.latvala@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>

Currently, the containers are used only to build IGT. We also want to
have an option to run IGT inside a container. For this, new build stage
is added.

v2: help touch-ups (Arek)

Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: Oleg Vasilev <oleg.vasilev@intel.com>