package/docker-engine: define <pkg>_GOMOD variable

In preparation for the go.mod integration, define the
DOCKER_ENGINE_GOMOD variable.

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/docker-engine/docker-engine.mk

@@ -12,6 +12,7 @@ DOCKER_ENGINE_LICENSE_FILES = LICENSE
 
 DOCKER_ENGINE_DEPENDENCIES = host-pkgconf
 DOCKER_ENGINE_SRC_SUBDIR = github.com/docker/docker
+DOCKER_ENGINE_GOMOD = github.com/docker/docker
 
 DOCKER_ENGINE_LDFLAGS = \
 	-X main.GitCommit=$(DOCKER_ENGINE_VERSION) \