New static configuration loading system.

Co-authored-by: Mathieu Lonjaret <mathieu.lonjaret@gmail.com>

pkg/cli/loader_env.go

@@ -0,0 +1,40 @@
+package cli
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/containous/traefik/pkg/config/env"
+	"github.com/containous/traefik/pkg/log"
+)
+
+// EnvLoader loads a configuration from all the environment variables prefixed with "TRAEFIK_".
+type EnvLoader struct{}
+
+// Load loads the command's configuration from the environment variables.
+func (e *EnvLoader) Load(_ []string, cmd *Command) (bool, error) {
+	return e.load(os.Environ(), cmd)
+}
+
+func (*EnvLoader) load(environ []string, cmd *Command) (bool, error) {
+	var found bool
+	for _, value := range environ {
+		if strings.HasPrefix(value, "TRAEFIK_") {
+			found = true
+			break
+		}
+	}
+
+	if !found {
+		return false, nil
+	}
+
+	if err := env.Decode(environ, cmd.Configuration); err != nil {
+		return false, fmt.Errorf("failed to decode configuration from environment variables: %v", err)
+	}
+
+	log.WithoutContext().Println("Configuration loaded from environment variables.")
+
+	return true, nil
+}