Add basic .devcontainer setup

.devcontainer/README.md

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+# Shared Dune Caching across DevContainers
+
+This container stores a dune cache on the host in `~/.cache/remote_dune`; this
+is shared between different containers.
+
+You can use `dune cache trim` commands inside a container to reduce its size, or
+simply remove the folder from the host as desired, as long as containers are not
+running.

.devcontainer/devcontainer.json

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+// For format details, see https://aka.ms/devcontainer.json.
+{
+  "name": "SkyLabs FM default",
+  "image":
+    "ghcr.io/skylabsai/workspace:fm-default",
+  "runArgs": [
+    "--platform",
+    "linux/amd64",
+    // We prefer --volume over "mounts" because it auto-creates the host folder.
+    "--volume",
+    "${localEnv:HOME}/.cache/remote_dune:/home/coq/.cache/dune:cached"
+  ],
+  "customizations": {
+    "vscode": {
+      "settings": {
+        "vsrocq.path": "/workspaces/workspace/_build/install/default/bin/vsrocqtop"
+      },
+      "extensions": [
+        "rocq-prover.vsrocq",
+        "ocamllabs.ocaml-platform",
+        "[email protected]",
+        "mr-konn.generic-input-method"
+      ]
+    }
+  }
+}