chore: add docs reminder comments for driver opts

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

driver/docker/driver.go

@@ -17,6 +17,8 @@ type Driver struct {
 	factory driver.Factory
 	driver.InitConfig
 
+	// if you add fields, remember to update docs:
+	// https://github.com/docker/docs/blob/main/content/build/drivers/docker.md
 	features    features
 	hostGateway hostGateway
 }