docs: add DOCKER_HOST guidance for non-default Docker socket paths (#26807)

## What

Add `DOCKER_HOST` guidance for non-default Docker socket paths to two
pages:

- `docs/install/docker.md`: expands the **Cannot connect to the Docker
daemon**
troubleshooting section with the `DOCKER_HOST` fix and how to persist it
to
  your shell startup file.
- `docs/admin/templates/troubleshooting.md`: adds a concise **Cannot
connect to
the Docker daemon** entry that cross-references the install guide for
the full
  steps.

## Why

`install/docker.md` previously documented only the default socket path
(`/var/run/docker.sock`). When Docker runs through a tool that uses a
per-user
socket, such as rootless Docker on Linux, or Colima, Podman, or Rancher
Desktop
on macOS, the daemon exposes its socket at a non-default path, so the
Coder
server cannot connect until `DOCKER_HOST` is set. The guidance frames
Colima as
one example, notes that default socket paths vary by tool, and persists
the
setting in a shell-agnostic way.

Generated by Coder Agents on behalf of @nickvigilante.

docs/admin/templates/troubleshooting.md

@@ -171,6 +171,13 @@ to optimize your templates based on this data.
 
 ![Workspace build timings UI](../../images/admin/templates/troubleshooting/workspace-build-timings-ui.png)
 
+## Cannot connect to the Docker daemon
+
+If a Docker-based template fails to provision with an error like `Cannot connect to the Docker daemon at unix:///var/run/docker.sock`, the Coder host cannot reach the Docker socket.
+Confirm that Docker is installed and running on the host.
+If you run Docker through rootless Docker, [Colima](https://colima.run), Podman, or a similar tool, the daemon may expose its socket at a non-default path, so set `DOCKER_HOST` to point at it.
+Refer to [Cannot connect to the Docker daemon](../../install/docker.md#cannot-connect-to-the-docker-daemon) for the full steps.
+
 ## Docker Workspaces on Raspberry Pi OS
 
 ### Unable to query ContainerMemory