Enable FUSE in local-dev Docker containers

In ContainerClient::createContainer(), set Privileged=true, CapAdd=[SYS_ADMIN],
Devices=[/dev/fuse], and SecurityOpt=[apparmor:unconfined] on the user's app
container. Mirrors what Cloudflare's production container runtime effectively
provides, so Workers that use FUSE (via e.g. the Containers FUSE binding)
work in local dev the way they work in production.

Also adds missing $Json.name annotations to docker_api::DeviceMapping fields
so they serialize as PathOnHost / PathInContainer / CgroupPermissions to
match the Docker API. Before this change no call site populated Devices,
so the missing annotations were latent.

Follows the same pattern as createSidecarContainer(), which already sets
CapAdd=[NET_ADMIN] for its own need at container-client.c++:1958-1960.

End-to-end reproduction (FUSE mount syscall actually succeeding inside the
container) in https://github.com/Ben2W/workerd-fuse-local-repro

Signed-off-by: Ben Werner <bewerner23@gmail.com>

Author: Ben2W

src/workerd/server/container-client.c++

@@ -1698,6 +1698,35 @@ kj::Promise<void> ContainerClient::createContainer(kj::StringPtr effectiveImage,
     }
   }
 
+  // Local-dev FUSE parity with production.
+  //
+  // Cloudflare's production container runtime exposes /dev/fuse to workers
+  // and grants the capabilities required to mount it. Locally we talk to a
+  // stock Docker daemon, which by default does not bind /dev/fuse into the
+  // container and does not grant CAP_SYS_ADMIN, so `open("/dev/fuse")` and
+  // the subsequent `mount("fuse", ...)` syscall fail with ENOENT / EPERM.
+  // Workers that use FUSE (e.g. the containers FUSE binding) therefore
+  // work in prod but break in local dev. Mirror what prod effectively
+  // provides by running the user's container privileged with /dev/fuse
+  // exposed. This only affects local development via workerd's Docker
+  // client; production is unaffected.
+  hostConfig.setPrivileged(true);
+  {
+    auto capAdd = hostConfig.initCapAdd(1);
+    capAdd.set(0, "SYS_ADMIN");
+  }
+  {
+    auto devices = hostConfig.initDevices(1);
+    auto fuseDev = devices[0];
+    fuseDev.setPathOnHost("/dev/fuse");
+    fuseDev.setPathInContainer("/dev/fuse");
+    fuseDev.setCgroupPermissions("rwm");
+  }
+  {
+    auto securityOpt = hostConfig.initSecurityOpt(1);
+    securityOpt.set(0, "apparmor:unconfined");
+  }
+
   auto response = co_await dockerApiRequest(network, kj::str(dockerPath), kj::HttpMethod::POST,
       kj::str("/containers/create?name=", containerName), codec.encode(jsonRoot));
 

src/workerd/server/docker-api.capnp

@@ -32,9 +32,9 @@ struct Docker {
   }
 
   struct DeviceMapping {
-    pathOnHost @0 :Text;
-    pathInContainer @1 :Text;
-    cgroupPermissions @2 :Text;
+    pathOnHost @0 :Text $Json.name("PathOnHost");
+    pathInContainer @1 :Text $Json.name("PathInContainer");
+    cgroupPermissions @2 :Text $Json.name("CgroupPermissions");
   }
 
   struct DeviceRequest {