Ensure Dockerfile stages use consistent Go version (#9462)

I noticed the tendermint image was running on Go 1.15. I assume that was just a missed search and replace when updating to go1.18.

Pull the go base image into a build arg so that the image is only defined once, and used consistently across all stages of the build.

#### PR checklist

- [x] Tests written/updated, or no tests needed
- [x] `CHANGELOG_PENDING.md` updated, or no changelog entry needed
- [x] Updated relevant documentation (`docs/`) and code comments, or no
      documentation updates needed

CHANGELOG_PENDING.md

@@ -27,6 +27,8 @@
 
 ### BUG FIXES
 
+- [docker] \#9462 ensure Docker image uses consistent version of Go
+
 ## v0.37.0
 
 Special thanks to external contributors on this release:

DOCKER/Dockerfile

@@ -1,5 +1,9 @@
+# Use a build arg to ensure that both stages use the same,
+# hopefully current, go version.
+ARG GOLANG_BASE_IMAGE=golang:1.18-alpine
+
 # stage 1 Generate Tendermint Binary
-FROM --platform=$BUILDPLATFORM golang:1.18-alpine as builder
+FROM --platform=$BUILDPLATFORM $GOLANG_BASE_IMAGE as builder
 RUN apk update && \
     apk upgrade && \
     apk --no-cache add make
@@ -8,7 +12,7 @@ WORKDIR /tendermint
 RUN TARGETPLATFORM=$TARGETPLATFORM make build-linux
 
 # stage 2
-FROM golang:1.15-alpine
+FROM $GOLANG_BASE_IMAGE
 LABEL maintainer="hello@tendermint.com"
 
 # Tendermint will be looking for the genesis file in /tendermint/config/genesis.json