Bump up Go docker image to 1.15.5 (#16744)

```release-notes
NONE
```

build/Dockerfile.buildenv

@@ -1,3 +1,3 @@
-FROM golang:1.14.6
+FROM golang:1.15.5
 
 RUN apt-get update && apt-get install -y make git apt-transport-https ca-certificates curl software-properties-common build-essential zip xmlsec1 jq