docker: bump to go 1.15.5

Author: changkun

docker/Dockerfile

@@ -2,7 +2,7 @@
 # All rights reserved. Use of this source code is governed
 # by a MIT license that can be found in the LICENSE file.
 
-FROM golang:1.15.3
+FROM golang:1.15.5
 WORKDIR /app
 COPY . .
 CMD ["/app/redir.app", "-s"]