Optimize the dockerfile file and add the function of customizing the ldif file

[ilanni2460]

Optimize the dockerfile file and add the function of customizing the ldif file

`
FROM gcr.io/distroless/java:8
MAINTAINER Josef (kwart) Cacek josef.cacek@gmail.com

COPY target/ldap-server.jar /ldap-server.jar

ENV LDIF_FILE=/ldap-server-users.ldif

COPY users.ldif /ldap-server-users.ldif

EXPOSE 389 636

ENTRYPOINT ["/usr/bin/java", "-jar", "/ldap-server.jar"]
CMD ["-a", "-p", "389", "-sp", "636", "-b", "0.0.0.0", "${LDIF_FILE}"]

`