Using just `kas-container build kas/<platform>.yml` is broken because
UEFI_CERT_FILE and UEFI_CAPSULE_CERT_FILE are not accessible from inside
of the /work/ directory.

`kas-container` is useful to replicate build issues in meta-ts CI
because they uses same software versions and environment.

Signed-off-by: Javier Tia <javier.tia@linaro.org>