Convert parameters to env variables

Consistent with other scripts in this repository,
make the input parameters env variable based.
This will enable future commits which will add
new input variables.

Change-Id: Ifb221a9ba84f0e2311b5c51e5de40f531d8831ad
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/build-unit-test-docker.sh b/build-unit-test-docker.sh
index 28f33d0..7607c0f 100755
--- a/build-unit-test-docker.sh
+++ b/build-unit-test-docker.sh
@@ -2,16 +2,16 @@
 #
 # Build the required docker image to run package unit tests
 #
-# Parameters:
-#   param1:  <optional, the name of the docker image to generate>
-#            default is openbmc/ubuntu-unit-test
-#   param2:  <optional, the distro to build a docker image against>
-#            default is ubuntu:artful
+# Script Variables:
+#   DOCKER_IMG_NAME:  <optional, the name of the docker image to generate>
+#                     default is openbmc/ubuntu-unit-test
+#   DISTRO:           <optional, the distro to build a docker image against>
+#                     default is ubuntu:bionic
 
 set -uo pipefail
 
-DOCKER_IMG_NAME=${1:-"openbmc/ubuntu-unit-test"}
-DISTRO=${2:-"ubuntu:bionic"}
+DOCKER_IMG_NAME=${DOCKER_IMG_NAME:-"openbmc/ubuntu-unit-test"}
+DISTRO=${DISTRO:-"ubuntu:bionic"}
 
 # Determine the architecture
 ARCH=$(uname -m)