[docker] Run as non-root user, fixes permissions. (#5702)

run-in-docker.sh

@@ -10,7 +10,9 @@ mkdir -p "${maven_cache_repo}"
 docker run --rm -it \
         -w /gen \
         -e GEN_DIR=/gen \
+        -e MAVEN_CONFIG=/var/maven/.m2 \
+        -u "$(id -u):$(id -u)" \
         -v "${PWD}:/gen" \
-        -v "${maven_cache_repo}:/root/.m2/repository" \
+        -v "${maven_cache_repo}:/var/maven/.m2/repository" \
         --entrypoint /gen/docker-entrypoint.sh \
         maven:3-jdk-7 "$@"