rewrite of exec command, use docker runner code

Author: TP Honey

drone/exec/env.go

@@ -63,15 +63,3 @@ func prefixedEnviron(environ []string) map[string]string {
 	}
 	return envs
 }
-
-// helper function combines one or more maps of environment
-// variables into a single map.
-func combineEnviron(env ...map[string]string) map[string]string {
-	c := map[string]string{}
-	for _, e := range env {
-		for k, v := range e {
-			c[k] = v
-		}
-	}
-	return c
-}