Add volume subpath property

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

05-services.md

@@ -1721,6 +1721,7 @@ services:
         target: /data
         volume:
           nocopy: true
+          subpath: sub
       - type: bind
         source: /var/run/postgres/postgres.sock
         target: /var/run/postgres/postgres.sock
@@ -1772,6 +1773,7 @@ expressed in the short form.
   - `selinux`: The SELinux re-labeling option `z` (shared) or `Z` (private)
 - `volume`: Configures additional volume options:
   - `nocopy`: Flag to disable copying of data from a container when a volume is created.
+  - `subpath`: Path inside a volume to mount instead of the volume root.
 - `tmpfs`: Configures additional tmpfs options:
   - `size`: The size for the tmpfs mount in bytes (either numeric or as bytes unit).
   - `mode`: The file mode for the tmpfs mount as Unix permission bits as an octal number.

schema/compose-spec.json

@@ -390,7 +390,8 @@
                   "volume": {
                     "type": "object",
                     "properties": {
-                      "nocopy": {"type": "boolean"}
+                      "nocopy": {"type": "boolean"},
+                      "subpath": {"type": "string"}
                     },
                     "additionalProperties": false,
                     "patternProperties": {"^x-": {}}