Add redis pool to registry webapp

Redis has been integrated with the web application for use with various
services. The configuraiton exposes connection details, timeouts and pool
parameters. Documentation has been updated accordingly.

A few convenience methods have been added to the context package to get loggers
with certain fields, exposing some missing functionality from logrus.

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

context/logger.go

@@ -45,6 +45,20 @@ func WithLogger(ctx context.Context, logger Logger) context.Context {
 	return context.WithValue(ctx, "logger", logger)
 }
 
+// GetLoggerWithField returns a logger instance with the specified field key
+// and value without affecting the context. Extra specified keys will be
+// resolved from the context.
+func GetLoggerWithField(ctx context.Context, key, value interface{}, keys ...interface{}) Logger {
+	return getLogrusLogger(ctx, keys...).WithField(fmt.Sprint(key), value)
+}
+
+// GetLoggerWithFields returns a logger instance with the specified fields
+// without affecting the context. Extra specified keys will be resolved from
+// the context.
+func GetLoggerWithFields(ctx context.Context, fields map[string]interface{}, keys ...interface{}) Logger {
+	return getLogrusLogger(ctx, keys...).WithFields(logrus.Fields(fields))
+}
+
 // GetLogger returns the logger from the current context, if present. If one
 // or more keys are provided, they will be resolved on the context and
 // included in the logger. While context.Value takes an interface, any key