Update UID/GID rather than create user

Author: Chuxel

containers/r/.devcontainer/Dockerfile

@@ -8,12 +8,11 @@ FROM rocker/r-apt:bionic
 # Avoid warnings by switching to noninteractive
 ENV DEBIAN_FRONTEND=noninteractive
 
-# This Dockerfile adds a non-root 'vscode' user with sudo access. However, for Linux,
-# this user's GID/UID must match your local user UID/GID to avoid permission issues
-# with bind mounts. Update USER_UID / USER_GID if yours is not 1000. See
-# https://aka.ms/vscode-remote/containers/non-root-user for details.
-ARG USERNAME=vscode
-ARG USER_UID=1001
+# The r-apt image comes with a base non-root 'docker' user which this Dockerfile
+# gives sudo access. However, for Linux, this user's GID/UID must match your local
+# user UID/GID to avoid permission issues with bind mounts. Update USER_UID / USER_GID 
+# if yours is not 1000. See https://aka.ms/vscode-remote/containers/non-root-user.
+ARG USER_UID=1000
 ARG USER_GID=$USER_UID
 
 # Configure apt and install packages
@@ -33,13 +32,13 @@ RUN apt-get update \
     && apt-get update \
     && apt-get -y install dotnet-runtime-2.1 \
     #
-    # Create a non-root user to use if preferred - see https://aka.ms/vscode-remote/containers/non-root-user.
-    && groupadd --gid $USER_GID $USERNAME \
-    && useradd -s /bin/bash --uid $USER_UID --gid $USER_GID -m $USERNAME \
+    # [Optional] Update a non-root user to match UID/GID - see https://aka.ms/vscode-remote/containers/non-root-user.
+    && if [ "$USER_GID" != "1000" ]; then groupmod docker --gid $USER_GID; fi \
+    && if [ "$USER_UID" != "1000" ]; then usermod --uid $USER_UID docker; fi \
     # [Optional] Add sudo support for the non-root user
     && apt-get install -y sudo \
-    && echo $USERNAME ALL=\(root\) NOPASSWD:ALL > /etc/sudoers.d/$USERNAME\
-    && chmod 0440 /etc/sudoers.d/$USERNAME \
+    && echo $USERNAME ALL=\(root\) NOPASSWD:ALL > /etc/sudoers.d/docker\
+    && chmod 0440 /etc/sudoers.d/docker \
     #
     # Clean up
     && apt-get autoremove -y \

containers/r/.devcontainer/devcontainer.json

@@ -5,7 +5,7 @@
 		// Uncomment the next line to use a non-root user. On Linux, this will prevent
 		// new files getting created as root, but you may need to update the USER_UID
 		// and USER_GID in .devcontainer/Dockerfile to match your user if not 1000.
-		// "-u", "vscode",
+		// "-u", "docker",
 
 		"--cap-add=SYS_PTRACE", "--security-opt", "seccomp=unconfined"
 	],