Fix missing addgroup/adduser commands in Debian slim image (#584)

Replace addgroup/adduser with groupadd/useradd which are available
in the debian:stable-slim base image. The addgroup and adduser commands
are part of the adduser package which is not installed in the slim image.

This fixes the build failure:
/bin/bash: line 1: addgroup: command not found

Continues fix for #582

Author: fank

docker/Dockerfile

@@ -54,8 +54,8 @@ RUN apt-get -q update \
     fi \
     && rm -rf /var/lib/apt/lists/*
 
-RUN addgroup --system --gid "$PGID" "$GROUP" \
-    && adduser --system --uid "$PUID" --gid "$PGID" --no-create-home --disabled-password --shell /bin/sh "$USER"
+RUN groupadd --system --gid "$PGID" "$GROUP" \
+    && useradd --system --uid "$PUID" --gid "$PGID" --no-create-home --shell /bin/sh "$USER"
 
 # version checksum of the archive to download
 ARG VERSION