Add org.label-schema.vcs-utc field to specification

rc1.md

@@ -72,6 +72,7 @@ All labels are OPTIONAL, however if present MUST be prefixed with the namespace
 | `url` | `org.label-schema.url="http://postgresql.org"` | URL of website with more information about the product or service provided by the container. |
 | `vcs-url` | `org.label-schema.vcs-url = "https://github.com/nginx/nginx"` | URL for the source code under version control from which this container image was built. |
 | `vcs-ref` | `org.label-schema.vcs-ref = "279FA63"` | Identifier for the version of the source code from which this image was built. For example if the version control system is git this is the SHA. |
+| `vcs-utc` | `org.label-schema.vcs-utc = "2006-01-02T15:04:05Z"` | UTC commit time in RFC 3339 format with second accuracy for the version of the source code from which this image was built. |
 | `vendor` | `org.label-schema.vendor = "Stark Industries"` | The organization that produces this image. |
 | `version` | `org.label-schema.version = "1.2.3"` `org.label-schema.version = "Beta4.2"` `org.label-schema.version = "1.2.2-dirty"` `org.label-schema.version = "my-test"` | Release identifier for the contents of the image. This is entirely up to the user and could be a numeric version number like 1.2.3, or a text label.<br> The version MAY match a label or tag in the source code repository.<br> You should make sure that only images that exactly reflect a version of code should have that version label. If Julia finds a version 0.7.1 in a repository she SHOULD be able to infer this matches version 0.7.1 of the associated code (and in particular, not 0.7.1 plus some later commits).<br> You SHOULD omit the version label, or use a marker like "dirty" or "test" to indicate when a container image does not match a labelled / tagged version of the code. |
 | `schema-version` | `org.label-schema.schema-version = "1.0"` | This label SHOULD be present to indicate the version of Label Schema in use. |