Expand build argument from environment when no value specified

Fixes #713

cmd/executor/cmd/root.go

@@ -55,6 +55,7 @@ var RootCmd = &cobra.Command{
 	Use: "executor",
 	PersistentPreRunE: func(cmd *cobra.Command, args []string) error {
 		if cmd.Use == "executor" {
+			resolveEnvironmentBuildArgs(opts.BuildArgs, os.Getenv)
 			if err := util.ConfigureLogging(logLevel); err != nil {
 				return err
 			}
@@ -200,6 +201,17 @@ func resolveDockerfilePath() error {
 	return errors.New("please provide a valid path to a Dockerfile within the build context with --dockerfile")
 }
 
+// resolveEnvironmentBuildArgs replace build args without value by the same named environment variable
+func resolveEnvironmentBuildArgs(arguments []string, resolver func(string) string) {
+	for index, argument := range arguments {
+		i := strings.Index(argument, "=")
+		if i < 0 {
+			value := resolver(argument)
+			arguments[index] = fmt.Sprintf("%s=%s", argument, value)
+		}
+	}
+}
+
 // copy Dockerfile to /kaniko/Dockerfile so that if it's specified in the .dockerignore
 // it won't be copied into the image
 func copyDockerfile() error {

cmd/executor/cmd/root_test.go

@@ -97,3 +97,55 @@ func TestIsUrl(t *testing.T) {
 		})
 	}
 }
+
+func TestResolveEnvironmentBuildArgs(t *testing.T) {
+	tests := []struct {
+		description               string
+		input                     []string
+		expected                  []string
+		mockedEnvironmentResolver func(string) string
+	}{
+		{
+			description: "replace when environment variable is present and value is not specified",
+			input:       []string{"variable1"},
+			expected:    []string{"variable1=value1"},
+			mockedEnvironmentResolver: func(variable string) string {
+				if variable == "variable1" {
+					return "value1"
+				}
+				return ""
+			},
+		},
+		{
+			description: "do not replace when environment variable is present and value is specified",
+			input:       []string{"variable1=value1", "variable2=value2"},
+			expected:    []string{"variable1=value1", "variable2=value2"},
+			mockedEnvironmentResolver: func(variable string) string {
+				return "unexpected"
+			},
+		},
+		{
+			description: "do not replace when environment variable is present and empty value is specified",
+			input:       []string{"variable1="},
+			expected:    []string{"variable1="},
+			mockedEnvironmentResolver: func(variable string) string {
+				return "unexpected"
+			},
+		},
+		{
+			description: "replace with empty value when environment variable is not present or empty and value is not specified",
+			input:       []string{"variable1", "variable2=value2"},
+			expected:    []string{"variable1=", "variable2=value2"},
+			mockedEnvironmentResolver: func(variable string) string {
+				return ""
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.description, func(t *testing.T) {
+			resolveEnvironmentBuildArgs(tt.input, tt.mockedEnvironmentResolver)
+			testutil.CheckDeepEqual(t, tt.expected, tt.input)
+		})
+	}
+}