refactor dockerfile

Dockerfile

@@ -3,26 +3,27 @@ FROM alpine:3.13.4
 ARG MARIADB_CLIENT_PACKAGE_VERSION=10.5.8-r0
 ARG OPENSSH_PACKAGE_VERSION=8.4_p1-r3
 ARG TINI_PACKAGE_VERSION=0.19.0-r0
+ARG USER=dump
+ENV SSHD_HOST_KEYS_DIR=/etc/ssh/host_keys
 RUN apk add --no-cache \
         mariadb-client=$MARIADB_CLIENT_PACKAGE_VERSION \
         openssh-server=$OPENSSH_PACKAGE_VERSION \
         tini=$TINI_PACKAGE_VERSION \
-    && adduser -S dump
+    && adduser -S "$USER" \
+    && mkdir "$SSHD_HOST_KEYS_DIR" \
+    && chown -c "$USER" "$SSHD_HOST_KEYS_DIR"
+VOLUME $SSHD_HOST_KEYS_DIR
 
 # RUN apk add --no-cache man openssh-doc=$OPENSSH_PACKAGE_VERSION
 
 COPY entrypoint.sh /
 ENTRYPOINT ["/sbin/tini", "--", "/entrypoint.sh"]
 COPY sshd_config /etc/ssh/sshd_config
-ENV SSHD_HOST_KEYS_DIR /etc/ssh/host_keys
+EXPOSE 2200/tcp
 ENV MYSQLDUMP_ARGS --help
 RUN chmod a=rx /entrypoint.sh \
     && chmod a=r /etc/ssh/sshd_config \
-    && sed -i 's#^\(dump:.*\):/sbin/nologin$#\1:/tmp/mysqldump.sh#' /etc/passwd \
-    && mkdir $SSHD_HOST_KEYS_DIR \
-    && chown dump $SSHD_HOST_KEYS_DIR
-VOLUME $SSHD_HOST_KEYS_DIR
+    && sed -i 's#^\(dump:.*\):/sbin/nologin$#\1:/tmp/mysqldump.sh#' /etc/passwd
 
-USER dump
-EXPOSE 2200/tcp
+USER $USER
 CMD ["/usr/sbin/sshd", "-D", "-e"]