Use go 1.15 (#867)

Dockerfile

@@ -4,7 +4,7 @@ ARG TARGET=server
 ARG GOPROXY
 
 # Build Temporal binaries
-FROM golang:1.14-alpine AS builder
+FROM golang:1.15-alpine AS builder
 
 RUN apk add --update --no-cache ca-certificates make curl git mercurial protobuf
 

docker/buildkite/Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.14
+FROM golang:1.15
 
 # Tried to set Python to ignore warnings due to the instructions at this link:
 # https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation