run-docker.sh: Allow multiple containers

By defining CONTAINER_NAME one can now build from multiple containers:

    CONTAINER_NAME=builder1 ./scripts/run-docker.sh
    CONTAINER_NAME=builder2 ./scripts/run-docker.sh
    ...

scripts/run-docker.sh

@@ -10,7 +10,7 @@ else
 fi
 
 IMAGE_NAME=termux/package-builder
-CONTAINER_NAME=termux-package-builder
+: ${CONTAINER_NAME:=termux-package-builder}
 
 USER=builder