Dockerfile - remove unused build stage used for building shapeshifter

Dockerfile

-FROM debian:bullseye-backports AS build
-RUN apt-get -q update \
-  && DEBIAN_FRONTEND=noninteractive apt-get install -qy --no-install-recommends \
-  build-essential \
-  pkg-config \
-  git \
-  ca-certificates \
-  golang-1.19-go \
-  && rm -rf /var/lib/apt/lists/*
-
-ENV GOPATH=/go
-WORKDIR $GOPATH
-
 FROM registry.git.autistici.org/ai3/docker/s6-overlay-lite:v12.0.0
 
 COPY kresd.key /usr/share/keyrings/kresd.gpg