Add support for configuration static logging fields

To allow flexibility in log message context information, this changeset
provides the ability to configure static fields that are included in the
context. Such fields can be set via configuration or environment variables.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

configuration/configuration.go

@@ -25,6 +25,10 @@ type Configuration struct {
 		// Formatter overrides the default formatter with another. Options
 		// include "text", "json" and "logstash".
 		Formatter string `yaml:"formatter"`
+
+		// Fields allows users to specify static string fields to include in
+		// the logger context.
+		Fields map[string]string `yaml:"fields"`
 	}
 
 	// Loglevel is the level at which registry operations are logged. This is