Add extra args config option

build.sh

-imagename="ndi-discovery"
+imagename="radialbog9/ndi-directory-service"
+extra_args="--push"
 
 # build <os/arch string> <name>
 function build() {
-    docker buildx build --platform $1 -f $2.dockerfile -t $imagename:$2 .
+    docker buildx build --platform $1 -f $2.dockerfile -t $imagename:$2 . $extra_args
 }
 
 # AMD64 image