docs(install): use CMD prefix in Docker healthcheck example

Docker Compose requires the test array to start with NONE, CMD, or CMD-SHELL.
The previous example was missing the prefix, so Compose treated the binary
path as the shell command, the healthcheck was rejected, and the container
was marked unhealthy — which causes Traefik (and similar) to skip it.

Closes #535

Amp-Thread-ID: https://ampcode.com/threads/T-019e7ae4-6862-760c-a3e7-239350eab71d
Co-authored-by: Amp <amp@ampcode.com>

docs/install/docker.md

@@ -57,7 +57,7 @@ services:
   headplane:
     image: ghcr.io/tale/headplane:latest
     healthcheck:
-      test: ["/bin/hp_healthcheck"]
+      test: ["CMD", "/bin/hp_healthcheck"]
       interval: 30s
       timeout: 5s
       start_period: 5s