Allow GC to discard content after successful pull and unpack

This commit adds a config flag for allowing GC to clean layer contents up after
unpacking these contents completed, which leads to deduplication of layer
contents between the snapshotter and the contnet store.

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>

docs/config.md

@@ -106,6 +106,10 @@ version = 2
     # This only works for runtime type "io.containerd.runtime.v1.linux".
     no_pivot = false
 
+    # discard_unpacked_layers allows GC to remove layers from the content store after
+    # successfully unpacking these layers to the snapshotter.
+    discard_unpacked_layers = false
+
     # default_runtime_name is the default runtime name to use.
     default_runtime_name = "runc"