docs: Clarify pull/build comments in migration

After pulling images, also run build for services with Dockerfiles.
This ensures build-based services have their images ready before
stopping the old service, minimizing downtime.

If build fails, abort the migration and leave the service running
on the old host.

Extract _migrate_service helper to reduce function complexity.

README.md

@@ -462,16 +462,16 @@ Update your Traefik config to use directory watching instead of a single file:
 
 There are many ways to run containers on multiple hosts. Here is where Compose Farm sits:
 
-| | Docker Contexts | K8s / Swarm | Ansible / Terraform | Portainer / Coolify | Compose Farm |
+| | Compose Farm | Docker Contexts | K8s / Swarm | Ansible / Terraform | Portainer / Coolify |
 |---|:---:|:---:|:---:|:---:|:---:|
-| No compose rewrites | ✅ | ❌ | ✅ | ✅ | ✅ |
-| Version controlled | ✅ | ✅ | ✅ | ❌ | ✅ |
-| State tracking | ❌ | ✅ | ✅ | ✅ | ✅ |
-| Auto-migration | ❌ | ✅ | ❌ | ❌ | ✅ |
-| Interactive CLI | ❌ | ❌ | ❌ | ❌ | ✅ |
-| Parallel execution | ❌ | ✅ | ✅ | ✅ | ✅ |
-| Agentless | ✅ | ❌ | ✅ | ❌ | ✅ |
-| High availability | ❌ | ✅ | ❌ | ❌ | ❌ |
+| No compose rewrites | ✅ | ✅ | ❌ | ✅ | ✅ |
+| Version controlled | ✅ | ✅ | ✅ | ✅ | ❌ |
+| State tracking | ✅ | ❌ | ✅ | ✅ | ✅ |
+| Auto-migration | ✅ | ❌ | ✅ | ❌ | ❌ |
+| Interactive CLI | ✅ | ❌ | ❌ | ❌ | ❌ |
+| Parallel execution | ✅ | ❌ | ✅ | ✅ | ✅ |
+| Agentless | ✅ | ✅ | ❌ | ✅ | ❌ |
+| High availability | ❌ | ❌ | ✅ | ❌ | ❌ |
 
 **Docker Contexts** — You can use `docker context create remote ssh://...` and `docker compose --context remote up`. But it's manual: you must remember which host runs which service, there's no global view, no parallel execution, and no auto-migration.
 

src/compose_farm/operations.py

@@ -130,6 +130,52 @@ async def _up_multi_host_service(
     return results
 
 
+async def _migrate_service(
+    cfg: Config,
+    service: str,
+    current_host: str,
+    target_host: str,
+    prefix: str,
+    *,
+    raw: bool = False,
+) -> CommandResult | None:
+    """Migrate a service from current_host to target_host.
+
+    Pre-pulls/builds images on target, then stops service on current host.
+    Returns failure result if migration prep fails, None on success.
+    """
+    console.print(
+        f"{prefix} Migrating from [magenta]{current_host}[/] → [magenta]{target_host}[/]..."
+    )
+    # Prepare images on target host before stopping old service to minimize downtime.
+    # Pull handles image-based services; build handles Dockerfile-based services.
+    # Each command is a no-op for the other type (exit 0, no work done).
+    pull_result = await run_compose(cfg, service, "pull", raw=raw)
+    if raw:
+        print()  # Ensure newline after raw output
+    if not pull_result.success:
+        err_console.print(
+            f"{prefix} [red]✗[/] Pull failed on [magenta]{target_host}[/], "
+            "leaving service on current host"
+        )
+        return pull_result
+    build_result = await run_compose(cfg, service, "build", raw=raw)
+    if raw:
+        print()  # Ensure newline after raw output
+    if not build_result.success:
+        err_console.print(
+            f"{prefix} [red]✗[/] Build failed on [magenta]{target_host}[/], "
+            "leaving service on current host"
+        )
+        return build_result
+    down_result = await run_compose_on_host(cfg, service, current_host, "down", raw=raw)
+    if raw:
+        print()  # Ensure newline after raw output
+    if not down_result.success:
+        return down_result
+    return None
+
+
 async def up_services(
     cfg: Config,
     services: list[str],
@@ -162,15 +208,11 @@ async def up_services(
         # If service is deployed elsewhere, migrate it
         if current_host and current_host != target_host:
             if current_host in cfg.hosts:
-                console.print(
-                    f"{prefix} Migrating from "
-                    f"[magenta]{current_host}[/] → [magenta]{target_host}[/]..."
+                failure = await _migrate_service(
+                    cfg, service, current_host, target_host, prefix, raw=raw
                 )
-                down_result = await run_compose_on_host(cfg, service, current_host, "down", raw=raw)
-                if raw:
-                    print()  # Ensure newline after raw output
-                if not down_result.success:
-                    results.append(down_result)
+                if failure:
+                    results.append(failure)
                     continue
             else:
                 err_console.print(