Add DOCKER_EXPERIMENTAL environment variable

The DOCKER_EXPERIMENTAL environment variable drives the activation of
the 'experimental' build tag.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>
Upstream-commit: ca6722f1c5e4e2017a71e33579d91ac0d9ea2a25
Component: engine

components/engine/docs/sources/reference/api/docker_remote_api_v1.19.md

@@ -1620,6 +1620,7 @@ Display system-wide information
             "Driver": "btrfs",
             "DriverStatus": [[""]],
             "ExecutionDriver": "native-0.1",
+            "ExperimentalBuild": false,
             "HttpProxy": "http://test:test@localhost:8080",
             "HttpsProxy": "https://test:test@localhost:8080",
             "ID": "7TRN:IPZB:QYBB:VPBQ:UMPP:KARE:6ZNR:XE6T:7EWV:PKF4:ZOJD:TPYS",