fx: drop the hardcoded rootless Docker ENV defaults

ENV always set DOCKER_HOST, so the ${DOCKER_HOST:-...} fallback in
/etc/profile could never use its default. The profile now sets both
variables, and a value from the environment is kept.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017jPxeenJt19PJStMMeb1Em

Author: claude

README.md

@@ -132,7 +132,7 @@ daemon automatically. The host must permit nested user namespaces — in a Coder
 run the workspace container with `privileged = true` (or the equivalent `--userns` setup), and call
 `dockerd-rootless-start` from `startup_script` so Docker is ready on boot.
 
-Both variables are defaults. If `DOCKER_HOST` is already set, the image and the script keep that
+Both variables are defaults. If `DOCKER_HOST` is already set, the profile and the script keep that
 value. The script starts no daemon if one answers there.
 
 ## Development

fx/Dockerfile

@@ -20,11 +20,6 @@ ENV GOROOT=/usr/local/go
 ENV GOPATH=/home/coder/go
 ENV PATH=$GOROOT/bin:$GOPATH/bin:$PATH
 
-# Rootless Docker (docker-in-docker without privileged root).
-# Defaults. The environment can override them.
-ENV XDG_RUNTIME_DIR=/run/user/1000
-ENV DOCKER_HOST=unix:///run/user/1000/docker.sock
-
 # T3 Code: opt out of telemetry by default
 ENV T3CODE_TELEMETRY_ENABLED=false