ContainerBackend accepts ignore patterns that name the mount point, which computerd then refuses at startup #195

Describe the bug

ContainerBackend checks ignore patterns in its constructor so that "a typo fails before a container starts rather than as a daemon that exits during startup" (ignore-assertion.ts). The check skips one step that computerd's resolveMountIgnore takes: computerd removes the mount point first (/workspace/dist becomes /dist), then runs the whole-mount check. The client never removes it. So a pattern that names the mount point passes the constructor, and then computerd refuses it and exits:

The two test suites, which the comment says "pin the same cases", have drifted too. computerd's whole-mount test includes "/workspace" and "/workspace/**", and the client's test does not.

Expected behavior

new ContainerBackend({ ..., ignore: ["/workspace/**"] }) throws the same "would make the whole mount local-only" error that ignore: ["/**"] does, without starting a container.

Steps to reproduce

On main (2f76387), with the two modules side by side:

import { checkIgnorePatterns } from "./packages/computer/src/backends/container/ignore-assertion.ts";
import { resolveMountIgnore } from "./packages/computerd/src/fuse/ignore.ts";

checkIgnorePatterns(["/workspace/**"]);              // no error
resolveMountIgnore(["/workspace/**"], "/workspace"); // MountIgnorePathError: ... would make the whole mount local-only

Proposed fix

Give checkIgnorePatterns the mount point and remove it the way computerd does, before the segment checks. The backend passes containerEnv?.MOUNT_POINT ?? "/workspace", which is the same value it puts in the container's env.

-export function checkIgnorePatterns(patterns: readonly string[]): void {
+export function checkIgnorePatterns(patterns: readonly string[], mountPoint = "/workspace"): void {
+  const root = mountPoint.replace(/\/+$/, "");
   for (const raw of patterns) {
 ...
-    const body = pattern.startsWith("!") ? pattern.slice(1) : pattern;
+    let body = pattern.startsWith("!") ? pattern.slice(1) : pattern;
 ...
     if (body.includes(",")) {
       fail("contains a comma, which separates patterns in MOUNT_IGNORE.");
     }
+    // computerd drops the mount point first: "/workspace/dist" is "/dist".
+    if (root !== "" && (body === root || body.startsWith(`${root}/`))) {
+      body = body.slice(root.length) || "/";
+    }

-    if (options.ignore !== undefined) checkIgnorePatterns(options.ignore);
+    if (options.ignore !== undefined) {
+      checkIgnorePatterns(options.ignore, options.containerEnv?.MOUNT_POINT ?? "/workspace");
+    }

Tests: "/workspace" and "/workspace/**" are added to the client's whole-mount list, so it matches computerd's. A new case covers /workspace/*, /workspace/**/*, a custom MOUNT_POINT, and confirms /workspace/dist is still accepted. On main these fail (2 failed, 24 passed). With the change, ignore-assertion.test.ts passes 26/26. I can share the full patch if useful.

Environment

@cloudflare/computer 0.4.0, main at 2f76387. Found by reading the code, then confirmed by calling both functions under Node 22. I have not deployed a container to see the startup failure end to end.