Docker: allow selection between root and non-root UID/GID at build time

Dockerfile

@@ -14,23 +14,26 @@ ENV GO111MODULE=on
 # build & install server
 RUN CGO_ENABLED=0 go build -tags netgo -ldflags "-X github.com/dutchcoders/transfer.sh/cmd.Version=$(git describe --tags) -a -s -w -extldflags '-static'" -o /go/bin/transfersh
 
-ARG PUID=1000 \
-    PGID=1000
+ARG PUID=5000 \
+    PGID=5000 \
+    RUNAS
 
 RUN mkdir -p /tmp/useradd && \
-    echo "transfersh:x:${PUID}:${PGID}::/nonexistent:/sbin/nologin" >> /tmp/useradd/passwd && \
-    echo "transfersh:!:::::::" >> /tmp/useradd/shadow && \
-    echo "transfersh:x:${PGID}:" >> /tmp/useradd/group && \
-    echo "transfersh:!::" >> /tmp/useradd/groupshadow
+    if [ ! -z "$RUNAS" ]; then \
+    echo "${RUNAS}:x:${PUID}:${PGID}::/nonexistent:/sbin/nologin" >> /tmp/useradd/passwd && \
+    echo "${RUNAS}:!:::::::" >> /tmp/useradd/shadow && \
+    echo "${RUNAS}:x:${PGID}:" >> /tmp/useradd/group && \
+    echo "${RUNAS}:!::" >> /tmp/useradd/groupshadow; fi
 
 FROM scratch AS final
 LABEL maintainer="Andrea Spacca <andrea.spacca@gmail.com>"
+ARG RUNAS
 
 COPY --from=build /tmp/useradd/* /etc/
-COPY --from=build --chown=transfersh  /go/bin/transfersh /go/bin/transfersh
+COPY --from=build --chown=${RUNAS}  /go/bin/transfersh /go/bin/transfersh
 COPY --from=build /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
 
-USER transfersh
+USER ${RUNAS}
 
 ENTRYPOINT ["/go/bin/transfersh", "--listener", ":8080"]