remove docker username argument

Dockerfile

@@ -11,6 +11,5 @@ RUN apt-get update && \
       llvm \
       scons
 
-ARG username
-RUN useradd $username
-USER $username
+RUN useradd rt
+USER rt

docker.bash

@@ -5,7 +5,7 @@ set -xe
 name="rt"
 image="$name-builder"
 
-docker build --tag "$image" --build-arg "username=$name" "$(dirname $0)"
+docker build --tag "$image" "$(dirname $0)"
 
 workdir="/home/$name"