diff --git a/builder.go b/builder.go
index e835c6d390..b22c853730 100644
--- a/builder.go
+++ b/builder.go
@@ -38,6 +38,9 @@ func (builder *Builder) mergeConfig(userConf, imageConf *Config) {
 	if userConf.MemorySwap == 0 {
 		userConf.MemorySwap = imageConf.MemorySwap
 	}
+	if userConf.CpuShares == 0 {
+		userConf.CpuShares = imageConf.CpuShares
+	}
 	if userConf.PortSpecs == nil || len(userConf.PortSpecs) == 0 {
 		userConf.PortSpecs = imageConf.PortSpecs
 	}