Environment variables with multiple '=' are not parsed correctly (#278)

* Provide failing test for an env variable with multiple '='

* Environment variables are split only in two parts

integration/dockerfiles/Dockerfile_test_env

@@ -1,6 +1,7 @@
 FROM gcr.io/google-appengine/debian9@sha256:1d6a9a6d106bd795098f60f4abb7083626354fa6735e81743c7f8cfca11259f0
 ENV hey hey
 ENV PATH /usr/local
+ENV testmultipleeq="this=is a=test string=with a=lot of=equals"
 ENV hey hello
 ENV first=foo second=foo2
 ENV third $second:/third

pkg/util/command_util.go

@@ -245,7 +245,7 @@ func UpdateConfigEnv(newEnvs []instructions.KeyValuePair, config *v1.Config, rep
 	// First, convert config.Env array to []instruction.KeyValuePair
 	var kvps []instructions.KeyValuePair
 	for _, env := range config.Env {
-		entry := strings.Split(env, "=")
+		entry := strings.SplitN(env, "=", 2)
 		kvps = append(kvps, instructions.KeyValuePair{
 			Key:   entry[0],
 			Value: entry[1],