style(dockerfile): fix command syntax in Dockerfile

images/actions/docker/Dockerfile

@@ -2,9 +2,8 @@ FROM git.kjan.de/actions/runner-basic:latest
 
 RUN sudo apk add --no-cache docker
 
-RUN addgroup -g 999 docker
+RUN sudo addgroup -g 999 docker
 
-RUN adduser -u 1001 -G docker -s /bin/sh -D runner
+RUN sudo adduser -u 1001 -G docker -s /bin/sh -D $USER
-USER runner
 
 ENV DOCKER_TLS_CERTDIR=""