docker: include git hash in docker-built binary

Dockerfile

 ## build ergo binary
 FROM golang:1.17-alpine3.13 AS build-env
 
-RUN apk add -U --force-refresh --no-cache --purge --clean-protected -l -u make
+RUN apk add -U --force-refresh --no-cache --purge --clean-protected -l -u make git
 
 # copy ergo source
 WORKDIR /go/src/github.com/ergochat/ergo