Edits to user guide + added filesystem section

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+title = "Filesystem"
+weight = 45
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+gVisor accesses the filesystem through a file proxy, called the Gofer. The gofer
+runs as a separate process, that is isolated from the sandbox. Gofer instances
+communicate with their respective sentry using the 9P protocol. For a more detailed
+explanation see [Overview > Gofer](../../architecture_guide/overview/#gofer).
+
+## Sandbox overlay
+
+To isolate the host filesystem from the sandbox, you can set a writable tmpfs overlay
+on top of the entire filesystem. All modifications are made to the overlay, keeping
+the host filesystem unmodified.
+
+> Note: All created and modified files are stored in memory inside the sandbox.
+
+To use the tmpfs overlay, add the following `runtimeArgs` to your Docker configuration
+(`/etc/docker/daemon.json`) and restart the Docker daemon:
+
+```json
+{
+    "runtimes": {
+        "runsc": {
+            "path": "/usr/local/bin/runsc",
+            "runtimeArgs": [
+                "--overlay"
+            ]
+       }
+    }
+}
+```
+
+## Shared root filesystem
+
+The root filesystem is where the image is extracted and is not generally modified
+from outside the sandbox. This allows for some optimizations, like skipping checks
+to determine if a directory has changed since the last time it was cached, thus
+missing updates that may have happened. If you need to `docker cp` files inside the
+root filesystem, you may want to enable shared mode. Just be aware that file system
+access will be slower due to the extra checks that are required.
+
+> Note: External mounts are always shared.
+
+To use set the root filesystem shared, add the following `runtimeArgs` to your Docker
+configuration (`/etc/docker/daemon.json`) and restart the Docker daemon:
+
+```json
+{
+    "runtimes": {
+        "runsc": {
+            "path": "/usr/local/bin/runsc",
+            "runtimeArgs": [
+                "--file-access=shared"
+            ]
+       }
+    }
+}
+```