Split install and configuration

Dockerfile

@@ -3,9 +3,10 @@
 FROM debian:stable
 
 ENV DATADIR=/usr/local/apache2/htdocs AUTHORIZED_KEYS_FILE=/authorized_keys USERID=33 GROUPID=33 OWNER=data
-RUN apt-get update \
- && apt-get install -y openssh-server \
- && rm -f /etc/ssh/ssh_host_* \
+RUN apt update \
+ && apt install -y openssh-server
+
+RUN rm -f /etc/ssh/ssh_host_* \
  && groupadd --non-unique --gid $GROUPID data \
  && useradd --non-unique --uid $USERID --gid $GROUPID --no-create-home --home-dir $DATADIR $OWNER \
  && mkdir -p "$DATADIR" \