forward DISTRO_APT_PREMIRRORS environment variable

This patch forwards the DISTRO_APT_PREMIRRORS environment variable into
the container when using the kas-container script. This variable is
commonly used to externally set mirrors for debian apt, e.g. in a CI
environment.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas-container

@@ -466,7 +466,7 @@ if [ -n "${KAS_REPO_REF_DIR}" ]; then
 fi
 
 for var in TERM KAS_DISTRO KAS_MACHINE KAS_TARGET KAS_TASK \
-           KAS_PREMIRRORS BB_NUMBER_THREADS PARALLEL_MAKE; do
+           KAS_PREMIRRORS DISTRO_APT_PREMIRRORS BB_NUMBER_THREADS PARALLEL_MAKE; do
 	if [ -n "$(eval echo \$${var})" ]; then
 		set -- "$@" -e "${var}=$(eval echo \"\$${var}\")"
 	fi