swagger: add BuildCache example to `/system/df`

Co-authored-by: Sebastiaan van Stijn <github@gone.nl>
Signed-off-by: Roman Volosatovs <roman.volosatovs@docker.com>

api/swagger.yaml

@@ -8339,7 +8339,29 @@ paths:
                   UsageData:
                     Size: 10920104
                     RefCount: 2
-              BuildCache: []
+              BuildCache:
+                -
+                  ID: "hw53o5aio51xtltp5xjp8v7fx"
+                  Parent: ""
+                  Type: "regular"
+                  Description: "pulled from docker.io/library/debian@sha256:234cb88d3020898631af0ccbbcca9a66ae7306ecd30c9720690858c1b007d2a0"
+                  InUse: false
+                  Shared: true
+                  Size: 0
+                  CreatedAt: "2021-06-28T13:31:01.474619385Z"
+                  LastUsedAt: "2021-07-07T22:02:32.738075951Z"
+                  UsageCount: 26
+                -
+                  ID: "ndlpt0hhvkqcdfkputsk4cq9c"
+                  Parent: "hw53o5aio51xtltp5xjp8v7fx"
+                  Type: "regular"
+                  Description: "mount / from exec /bin/sh -c echo 'Binary::apt::APT::Keep-Downloaded-Packages \"true\";' > /etc/apt/apt.conf.d/keep-cache"
+                  InUse: false
+                  Shared: true
+                  Size: 51
+                  CreatedAt: "2021-06-28T13:31:03.002625487Z"
+                  LastUsedAt: "2021-07-07T22:02:32.773909517Z"
+                  UsageCount: 26
         500:
           description: "server error"
           schema: