fx: respect an externally provided DOCKER_HOST

The rootless Docker setup hardcoded DOCKER_HOST in /etc/profile and in
dockerd-rootless-start, clobbering any value supplied by the environment
(workspace template, docker run -e, mounted host socket, remote daemon).

Both now treat the rootless socket as a fallback only. The startup script
skips starting a daemon when one already answers at DOCKER_HOST, derives
XDG_RUNTIME_DIR from a provided unix .../docker.sock so dockerd and the CLI
agree on the path, and fails with a clear message for non-unix hosts it
cannot serve.

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

Author: claude

README.md

@@ -132,6 +132,9 @@ 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
+value. The script starts no daemon if one answers there.
+
 ## Development
 
 Images are built and tested per template with the provided `Makefile`. `DOCKER_VARIANT` selects the

fx/Dockerfile

@@ -20,7 +20,8 @@ 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)
+# 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
 
@@ -127,7 +128,7 @@ RUN \
     echo '' >> /etc/profile && \
     echo '# Rootless Docker' >> /etc/profile && \
     echo 'export XDG_RUNTIME_DIR="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}"' >> /etc/profile && \
-    echo 'export DOCKER_HOST="unix://${XDG_RUNTIME_DIR}/docker.sock"' >> /etc/profile && \
+    echo 'export DOCKER_HOST="${DOCKER_HOST:-unix://${XDG_RUNTIME_DIR}/docker.sock}"' >> /etc/profile && \
     # OWNERSHIP (coder can upgrade tools) ######################################
     chown coder:coder /usr/local/bin/claude /usr/local/bin/opencode && \
     chown -R coder:coder /home/coder/go && \

fx/dockerd-rootless-start.sh

@@ -8,24 +8,39 @@
 #     dockerd-rootless-start
 #     docker run --rm hello-world
 #
+# A DOCKER_HOST from the environment is used if set.
+#
 # Note: the host must allow user namespaces. In Coder, run the container
 # with --userns-host or the equivalent template option.
 set -euo pipefail
 
 uid="$(id -u)"
-export XDG_RUNTIME_DIR="${XDG_RUNTIME_DIR:-/run/user/${uid}}"
-export DOCKER_HOST="unix://${XDG_RUNTIME_DIR}/docker.sock"
+export DOCKER_HOST="${DOCKER_HOST:-unix://${XDG_RUNTIME_DIR:-/run/user/${uid}}/docker.sock}"
+
+if docker info >/dev/null 2>&1; then
+    echo "docker daemon already reachable at ${DOCKER_HOST}"
+    exit 0
+fi
+
+# dockerd-rootless.sh listens on ${XDG_RUNTIME_DIR}/docker.sock. Set the runtime
+# dir from DOCKER_HOST so the daemon and the CLI use the same socket.
+case "$DOCKER_HOST" in
+    unix://*/docker.sock)
+        socket="${DOCKER_HOST#unix://}"
+        export XDG_RUNTIME_DIR="$(dirname "$socket")"
+        ;;
+    *)
+        echo "no daemon at DOCKER_HOST=${DOCKER_HOST}" >&2
+        echo "rootless dockerd needs a unix .../docker.sock path; unset DOCKER_HOST for the default" >&2
+        exit 1
+        ;;
+esac
 
 # /run/user/<uid> lives on tmpfs and is recreated on every container start.
 if [ ! -d "$XDG_RUNTIME_DIR" ]; then
     sudo install -d -m 0700 -o "$uid" -g "$(id -g)" "$XDG_RUNTIME_DIR"
 fi
 
-if docker info >/dev/null 2>&1; then
-    echo "rootless dockerd already running at ${DOCKER_HOST}"
-    exit 0
-fi
-
 echo "starting rootless dockerd ..."
 nohup dockerd-rootless.sh >/tmp/dockerd-rootless.log 2>&1 &