docs(docs/install): strengthen Linux-only requirement on Docker install page (#25742)

Closes DOCS-68.

Promotes the existing "Linux only" guidance on `docs/install/docker.md`
from an easy-to-miss bullet point to a prominent `[!IMPORTANT]` callout,
and briefly states *why* the page is Linux-only so macOS readers do not
waste time on the `getent` / `--group-add` snippets.

## Why this re-scope vs. the original ticket

The original DOCS-68 scope was "add a macOS `getent` alternative". On
inspection, that framing has three problems:

1. The Requirements section already says "A Linux machine. For macOS
devices, start Coder using the standalone binary," so macOS users are
already redirected. The signal just lives in a bullet that is easy to
overlook.
2. The `--group-add $DOCKER_GROUP` mechanism that drives the `getent`
call is Linux-specific. macOS Docker runtimes (Docker Desktop, Colima,
Rancher Desktop, Podman) use a VM and forward the socket differently;
the flag does not translate cleanly to any of them.
3. Defining a canonical macOS Docker path is the scope of
[DEVREL-22](https://linear.app/codercom/issue/DEVREL-22) (recommend
Colima / Rancher / Podman alternatives in the Quick Start guide).
DOCS-68 should not pre-empt that work.

This PR narrows the fix to making the existing macOS guidance
unmissable. A real macOS Docker install path can come as a separate
follow-up once DEVREL-22 lands and the recommended runtime is settled.

<details>
<summary>Decision log</summary>

* **(A) Close DOCS-68 as absorbed by DEVREL-22.** Rejected — the install
page still has a discoverability problem that DEVREL-22 (Quick Start)
will not fix.
* **(B) Re-scope DOCS-68 to a narrow today-fix (this PR).** Selected.
* **(C) Defer DOCS-68 until DEVREL-22 lands.** Rejected — the install
page is shipping the weaker guidance every day until then.

</details>

> [!NOTE]
> This is a docs-only change. No product code was modified.

---

*Generated by Coder Agents on behalf of @nickvigilante.*

docs/install/docker.md

@@ -8,11 +8,16 @@ You can install and run Coder using the official Docker images published on
 - Docker. See the
   [official installation documentation](https://docs.docker.com/install/).
 
-- A Linux machine. For macOS devices, start Coder using the
-  [standalone binary](./cli.md).
+- A Linux host.
 
 - 2 CPU cores and 4 GB memory free on your machine.
 
+> [!IMPORTANT]
+> This guide is for **Linux** hosts only. The `getent` and `--group-add`
+> Docker socket patterns used below are Linux-specific and do not translate
+> cleanly to macOS Docker runtimes. For macOS, install Coder using the
+> [standalone binary](./cli.md) instead.
+
 <div class="tabs">
 
 ## Install Coder via `docker compose`