Configure log4j for logstash using dockerfile

I recently started working on logstash. I am running it in docker and I want to read its logs using 'docker logs <container_name>' command. But I am getting error

log4j:WARN No appenders could be found for logger (org.apache.http.client.protocol.RequestAuthCache).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See for more info.

My dockerfile is as below, please let me know how to configure log4j using dockerfile.

FROM debian:jessie
ENV DEBIAN_FRONTEND noninteractive
RUN echo "deb jessie-backports main" >> /etc/apt/sources.list && \
    rm -rf /var/lib/apt/lists/* && \
    apt-get -y update && \
    apt-get -y upgrade && \
    apt-get -y --force-yes install sudo wget logrotate texinfo build-essential openjdk-7-jdk ant git zip && \
    apt-get autoclean && \
    apt-get autoremove && \
    rm -rf /var/lib/apt/lists/*

# Install logstash package
RUN wget --quiet && \
    dpkg -i ./logstash_2.3.0-1_all.deb && \
    rm ./logstash_2.3.0-1_all.deb

ENV PATH /opt/logstash/bin:$PATH
CMD ["/usr/bin/sudo", "-E", "-u", "logstash", "/opt/logstash/bin/logstash", "-f",  "/etc/logstash/logstash.conf"]
EXPOSE 5000-5100/tcp 5000-5100/udp

# Config ls
ADD logstash.conf /etc/logstash/logstash.conf

# Configuring log4j
ADD /etc/logstash/config/

What's in What if you remove its ADD stanza from the dockerfile?

I tried removing ADD stanza but it did not work.
Contents of

log4j.rootLogger=INFO, console, socketappender

PATTERN=[%d{ISO8601}][%-5p][%-25c] %m%n


Can you please let me know what is wrong with my configuration?
Thank you in advance!