Allow for the user to override the IMAGE_NAME shell variable

So that GitLab CI script (or any user, really) can customize the name of
the resulting Docker image.

Changelog: None

Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

docker-build

@@ -16,6 +16,6 @@
 
 set -e
 
-IMAGE_NAME=mender-convert
+IMAGE_NAME=${IMAGE_NAME:-mender-convert}
 
 eval docker build . -t ${IMAGE_NAME}

docker-mender-convert

@@ -16,7 +16,7 @@
 
 set -e
 
-IMAGE_NAME=mender-convert
+IMAGE_NAME=${IMAGE_NAME:-mender-convert}
 
 MENDER_CONVERT_DIR="$(pwd)"
 mkdir -p output