Force linux/amd64 platform usage for mock service (#8)

So apple silicon users can use it. We might want to publish a
`linux/arm64` at some point, but this is fine to start with.

Author: redox

README.md

@@ -140,6 +140,10 @@ pip install -e ".[dev]"
 pytest tests/
 ```
 
+The mock service container is forced to run as `linux/amd64` to keep parity with CI
+and to work reliably on Apple Silicon machines (Docker Desktop will handle
+emulation automatically).
+
 **Using an existing service (CI/production):**
 
 ```bash

tests/conftest.py

@@ -42,8 +42,11 @@ def __init__(
         self,
         image: str = "ghcr.io/altertable-ai/altertable-mock:latest",
         port: int = 15002,
+        platform: str = "linux/amd64",
     ):
-        super().__init__(image)
+        # Force the mock service to run on linux/amd64 so Apple Silicon hosts use the
+        # correct architecture (via emulation when needed).
+        super().__init__(image, platform=platform)
         self.port = port
         self.with_exposed_ports(port)
         self.with_env("ALTERTABLE_MOCK_FLIGHT_PORT", str(port))