sbx: use relative links for internal docs references

Compose, Engine, Desktop, and Docker Agent pages live on docs.docker.com,
so link to them with source-relative /manuals/ paths instead of absolute
https://docs.docker.com URLs. This matches the dominant convention in the
repo and lets Hugo resolve and link-check the references.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

content/manuals/ai/sandboxes/agents/docker-agent.md

@@ -7,7 +7,7 @@ description: |
 keywords: docker sandboxes, docker agent, openai, anthropic, sbx
 ---
 
-Official documentation: [Docker Agent](https://docs.docker.com/ai/docker-agent/)
+Official documentation: [Docker Agent](/manuals/ai/docker-agent/_index.md)
 
 ## Quick start
 

content/manuals/ai/sandboxes/customize/templates.md

@@ -82,7 +82,7 @@ $ sbx run claude --template docker.io/docker/sandbox-templates:claude-code
 ### Build a custom template
 
 Building a custom template requires
-[Docker Desktop](https://docs.docker.com/desktop/).
+[Docker Desktop](/manuals/desktop/_index.md).
 
 Write a Dockerfile that extends one of the base images. Pick the variant
 that matches the agent you plan to run. For example, extend `claude-code`

content/manuals/ai/sandboxes/security/isolation.md

@@ -57,7 +57,7 @@ Mounting your host Docker socket into a container would give the agent full
 access to your environment.
 
 Docker Sandboxes avoid this by running a separate [Docker
-Engine](https://docs.docker.com/engine/) inside the sandbox environment, isolated from
+Engine](/manuals/engine/_index.md) inside the sandbox environment, isolated from
 your host. When the agent runs `docker build` or `docker compose up`, those
 commands execute against that engine. The agent has no path to your host Docker
 daemon.

content/manuals/ai/sandboxes/usage.md

@@ -246,7 +246,7 @@ packages persist for the sandbox's lifetime. For teams or repeated setups,
 see [Customize](customize/) for reusable templates and declarative kits.
 
 Agents can also build Docker images, run containers, and use
-[Compose](https://docs.docker.com/compose/). Everything runs inside the sandbox's private Docker
+[Compose](/manuals/compose/_index.md). Everything runs inside the sandbox's private Docker
 daemon, so containers started by the agent never appear in your host's
 `docker ps`. When you remove the sandbox, all images, containers, and volumes
 inside it are deleted with it.