Use shared volume for devcontainer uv cache

Instead of mounting the user's local cache. For the first use this will
be slower (if the user has previously used uv) but using a separate
volume means links within the cache referring to devcontainer locations
will not interfere with uv being run from outside the container.

Using a named volume means devcontainers for multiple projects can share
the same cache.

Author: tpoliaw

.devcontainer/devcontainer.json

@@ -56,9 +56,9 @@
         },
         // Keep a persistent cross container cache for uv
         {
-            "source": "${localEnv:HOME}/.cache/uv",
+            "source": "persistent-uv-cache",
             "target": "/root/.cache/uv",
-            "type": "bind"
+            "type": "volume"
         },
         // Use a volume mount for the uv venv so it is local to the container
         {