docker: bump golang in /oldstable/build/alpine-x64

Bumps golang from 1.20.11-alpine3.18 to 1.20.12-alpine3.18.

---
updated-dependencies:
- dependency-name: golang
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

oldstable/build/alpine-x64/Dockerfile

@@ -7,7 +7,7 @@
 
 # https://hub.docker.com/_/golang
 
-FROM golang:1.20.11-alpine3.18
+FROM golang:1.20.12-alpine3.18
 
 # https://docs.github.com/en/packages/learn-github-packages/connecting-a-repository-to-a-package
 LABEL org.opencontainers.image.source="https://github.com/atc0005/go-ci"