LAT docker file downloads the installer from a hard-coded URL that
points to https://mirror.starlingx.windriver.com . Allow users to
override this location by defining STX_MIRROR_URL in the host
environment. By default, guess mirror location from stx.conf.
TESTS
==========================
* Rebuild LAT container and make sure it uses the mirror URL from
stx.conf
* Set STX_MIRROR_URL prior to calling stx-init-env and make sure it
gets picked up by the docker file
Story: 2010055
Task: 49883
Signed-off-by: Davlet Panech <davlet.panech@windriver.com>
Change-Id: Id8ea88407f74003db934337efd574451658633d8
44ccc58f55