new dockerfile based on chainguard images

Signed-off-by: Daniel Thamdrup <dallemon@protonmail.com>

Dockerfile

@@ -1,7 +1,6 @@
+# syntax=docker/dockerfile:latest
 ## build ergo binary
-FROM docker.io/golang:1.21-alpine AS build-env
-
-RUN apk upgrade -U --force-refresh --no-cache && apk add --no-cache --purge --clean-protected -l -u make git
+FROM cgr.dev/chainguard/go:latest AS build-env
 
 # copy ergo source
 WORKDIR /go/src/github.com/ergochat/ergo
@@ -15,34 +14,33 @@ RUN sed -i 's/^\(\s*\)\"127.0.0.1:6667\":.*$/\1":6667":/' /go/src/github.com/erg
 # compile
 RUN make install
 
+## tmp container for collecting files
+FROM scratch AS tmp
+
+# collect all files
+COPY --from=build-env /root/go/bin/ergo \
+                      /go/src/github.com/ergochat/ergo/default.yaml \
+                      /go/src/github.com/ergochat/ergo/distrib/docker/run.sh \
+                      /ircd-bin/
+COPY --from=build-env /go/src/github.com/ergochat/ergo/languages /ircd-bin/languages/
+COPY --from=build-env /go/src/github.com/ergochat/ergo/ergo.motd /ircd/ergo.motd
+
 ## build ergo container
-FROM docker.io/alpine:3.13
+FROM cgr.dev/chainguard/busybox:latest AS runtime
 
 # metadata
-LABEL maintainer="Daniel Oaks <daniel@danieloaks.net>,Daniel Thamdrup <dallemon@protonmail.com>" \
+LABEL maintainer="Daniel Oaks <daniel@danieloaks.net>,Daniel Thamdrup <danielthamdrup@pm.me>" \
       description="Ergo is a modern, experimental IRC server written in Go"
 
 # standard ports listened on
 EXPOSE 6667/tcp 6697/tcp
 
 # ergo itself
-COPY --from=build-env /go/bin/ergo \
-                      /go/src/github.com/ergochat/ergo/default.yaml \
-                      /go/src/github.com/ergochat/ergo/distrib/docker/run.sh \
-                      /ircd-bin/
-COPY --from=build-env /go/src/github.com/ergochat/ergo/languages /ircd-bin/languages/
+COPY --from=tmp --chown=nonroot:nonroot / /
 
 # running volume holding config file, db, certs
 VOLUME /ircd
 WORKDIR /ircd
 
-# default motd
-COPY --from=build-env /go/src/github.com/ergochat/ergo/ergo.motd /ircd/ergo.motd
-
 # launch
 ENTRYPOINT ["/ircd-bin/run.sh"]
-
-# # uncomment to debug
-# RUN apk add --no-cache bash
-# RUN apk add --no-cache vim
-# CMD /bin/bash