Option to configure proxy cache TTL

Currently when registry is run as proxy it tries to cleanup unused blobs
from its cache after 7 days which is hard-coded. This PR makes that
value configurable.

Co-authored-by: Shiming Zhang <wzshiming@foxmail.com>
Co-authored-by: Manish Tomar <manish.tomar@docker.com>
Signed-off-by: Shiming Zhang <wzshiming@foxmail.com>

docs/configuration.md

@@ -299,6 +299,7 @@ proxy:
   remoteurl: https://registry-1.docker.io
   username: [username]
   password: [password]
+  ttl: 168h
 compatibility:
   schema1:
     signingkeyfile: /etc/registry/key.json
@@ -1184,6 +1185,7 @@ proxy:
   remoteurl: https://registry-1.docker.io
   username: [username]
   password: [password]
+  ttl: 168h
 ```
 
 The `proxy` structure allows a registry to be configured as a pull-through cache
@@ -1197,6 +1199,7 @@ is unsupported.
 | `remoteurl`| yes     | The URL for the repository on Docker Hub.             |
 | `username` | no      | The username registered with Docker Hub which has access to the repository. |
 | `password` | no      | The password used to authenticate to Docker Hub using the username specified in `username`. |
+| `ttl`      | no      | Expire proxy cache configured in "storage" after this time. Cache 168h(7 days) by default, set to 0 to disable cache expiration, The suffix is one of `ns`, `us`, `ms`, `s`, `m`, or `h`. If you specify a value but omit the suffix, the value is interpreted as a number of nanoseconds. |
 
 
 To enable pulling private repositories (e.g. `batman/robin`) specify the

docs/recipes/mirror.md

@@ -91,6 +91,7 @@ proxy:
   remoteurl: https://registry-1.docker.io
   username: [username]
   password: [password]
+  ttl: 168h
 ```
 
 > **Warning**: If you specify a username and password, it's very important to