docker: Allow passing $DOCKER_USER to set the username in the container

Needs to be reflected in the documentation and the dockerhub description now.

Closes #881

ci/release-image/entrypoint.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env sh
+set -eu
+
+if [ "${DOCKER_USER-}" ]; then
+  echo "$DOCKER_USER ALL=(ALL) NOPASSWD:ALL" | sudo tee -a /etc/sudoers.d/nopasswd > /dev/null
+  sudo usermod --login "$DOCKER_USER" \
+    --move-home --home "/home/$DOCKER_USER" \
+    coder
+  sudo groupmod -n "$DOCKER_USER" coder
+
+  sudo sed -i "/coder/d" /etc/sudoers.d/nopasswd
+  sudo sed -i "s/coder/$DOCKER_USER/g" /etc/fixuid/config.yml
+  export HOME="/home/$DOCKER_USER"
+fi
+
+# This isn't set by default.
+export USER="$(whoami)"
+dumb-init fixuid -q /usr/bin/code-server "$@"