Merge pull request #1814 from slingamn/alpine_downgrade

docker: downgrade alpine to 3.13 again

Dockerfile

 ## build ergo binary
-FROM golang:1.17-alpine AS build-env
+FROM golang:1.17-alpine3.13 AS build-env
 
 RUN apk add -U --force-refresh --no-cache --purge --clean-protected -l -u make