MM-16725 Respect env var overrides when setting config (#11821)

* Respect env var overrides when setting config

* Use strings.NewReader

config/common_test.go

@@ -1,6 +1,7 @@
 package config_test
 
 import (
+	"os"
 	"testing"
 
 	"github.com/mattermost/mattermost-server/config"
@@ -134,5 +135,29 @@ func TestMergeConfigs(t *testing.T) {
 	})
 }
 
+func TestConfigEnvironmentOverrides(t *testing.T) {
+	base, err := config.NewMemoryStore()
+	require.NoError(t, err)
+	originalConfig := &model.Config{}
+	originalConfig.ServiceSettings.SiteURL = newString("http://notoverriden.ca")
+
+	os.Setenv("MM_SERVICESETTINGS_SITEURL", "http://overridden.ca")
+	defer os.Unsetenv("MM_SERVICESETTINGS_SITEURL")
+
+	t.Run("loading config should respect environment variable overrides", func(t *testing.T) {
+		err := base.Load()
+		require.NoError(t, err)
+
+		assert.Equal(t, "http://overridden.ca", *base.Get().ServiceSettings.SiteURL)
+	})
+
+	t.Run("setting config should respect environment variable overrides", func(t *testing.T) {
+		_, err := base.Set(originalConfig)
+		require.NoError(t, err)
+
+		assert.Equal(t, "http://overridden.ca", *base.Get().ServiceSettings.SiteURL)
+	})
+}
+
 func newBool(b bool) *bool       { return &b }
 func newString(s string) *string { return &s }