[wip] implement storage

For starters, document what we'll do.

Signed-off-by: Serge Hallyn <[email protected]>

docs/storage.md

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+# Storage for targes
+
+Following is an example manifest.yaml showing how to specify storage
+for targets:
+
+```
+storage:
+  - label: zot-data
+    persistent: true
+    nsgroup: "zot"
+    size: 30G
+  - label: zot-config
+    persistent: true
+    nsgroup: "zot"
+    size: 1G
+  - label: zot-tmp
+    persistent: false
+    nsgroup: "zot"
+    size: 1G
+  - label: nginx-data
+    persistent: true
+    nsgroup: "zot"
+    size: 1G
+targets:
+  - service_name: zot
+    source: docker://zothub.io/machine/bootkit/demo-zot:0.0.4-squashfs
+    version: 1.0.0
+    nsgroup: zot
+    storage:
+      - dest: /zot
+        label: zot-data
+      - dest: /etc/zot
+        label: zot-config
+      - dest: /tmp
+        label: zot-tmp
+  - service_name: nginx
+    source: docker://zothub.io/machine/bootkit/demo-nginx:0.0.4-squashfs
+    version: 1.0.0
+    nsgroup: zot
+    storage:
+      - dest: /data/zot
+        label: zot-data
+      - dest: /var/lib/www
+        label: nginx-data
+```
+
+When a target starts up, its rootfs is an overlay of a writeable tmpfs
+over the source OCI image (which itself is an overlay of dmverity-protected
+squashfs images).  The writeable overlays are all in a shared partition
+mounted at /scratch-writes.  In order to provide persistent storage
+across boots, shared storage between containers, or a larger private
+ephemeral storage which does not risk filling up /scratch-writes,
+extra storage can be requested.
+
+In the above example, four additional storage volumes are requested.  The
+30G volume called zot-data will be persistent, so its contents will be
+saved across boots. In contrast,  zot-tmp is not persistent, so its contents
+will be deleted across reboots.  All four are in the 'nsgroup zot', which
+both of the targets, zot and nginx, run in.  The nsgroup is a named
+user namespace mapping, so uid 0 will be represented by the same host
+uid (for instance 100000) for all.
+
+Note that if nginx were not placed into nsgroup 'zot', it would still
+be able to mount zot-data, however all files would appear as
+owned by nobody:nogroup, and nginx would get the world access rights.
+
+Each target now has an optional storage section, where it can
+specify which volumes it should mount, and where.
+
+On boot, the machine will first create the storage volumes, and uid-shift
+them if needed.  If a non-persistent volume already exists, it will be
+deleted and recreated.
+
+All storage volumes are created as ext4 filesystems.