what about the ability to configure the port via an environment variable?

[f1-outsourcing]

I know you can do this with the docker run (i think). But for other orchestration platforms like mesos it could be handy.

[wglambert]

The port publishing feature is done through the Docker engine, not the container's initialization.

So even if you mount Docker's socket and make a script to tell Docker to publish the port; environment variables wouldn't be able to affect its own running container since it's only "when you create a container"

[f1-outsourcing]

I am currently doing it like this

Dockerfile

FROM tomcat:8-slim

WORKDIR ${CATALINA_HOME}

ENV TOMCAT_USER="nobody" \
    TOMCAT_UID=99 \
    TOMCAT_HTTP_PORT="8080" \
    TOMCAT_HTTPS_PORT="8443" \
    OPENFIRE_SERVER="192.168.122.250"

# clean up examples
RUN rm -rf ${CATALINA_HOME}/webapps/*

# config changes
RUN groupadd $TOMCAT_USER -g $TOMCAT_UID \
    && chmod g+w ${CATALINA_HOME}/temp ${CATALINA_HOME}/webapps \
    && chgrp ${TOMCAT_USER} ${CATALINA_HOME}/temp ${CATALINA_HOME}/webapps \
    && sed -i '/<Connector port="8080"/a \               Server=" "' ${CATALINA_HOME}/conf/server.xml \
    && sed -i 's/"8080"/"${envHttpPort}"/' ${CATALINA_HOME}/conf/server.xml \
    && sed -i 's/"8443"/"${envHttpsPort}"/' ${CATALINA_HOME}/conf/server.xml \
    && sed -i 's/shutdown="SHUTDOWN"/shutdown="${envShutdown}"/g' ${CATALINA_HOME}/conf/server.xml

COPY entrypoint.sh /sbin/entrypoint.sh

EXPOSE ${TOMCAT_HTTP_PORT}/tcp

CMD ["/sbin/entrypoint.sh"]

And in entrypoint.sh

export TOMCAT_SHUTDOWN=$(cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 18 | head -n 1)
export JAVA_OPTS="$JAVA_OPTS -DenvHttpPort=$TOMCAT_HTTP_PORT -DenvHttpsPort=$TOMCAT_HTTPS_PORT -DenvShutdown=$TOMCAT_SHUTDOWN 

As I wrote, there is more than just docker.

[tianon]

As you've pointed out, it requires modification of XML configuration to change the port -- we currently do not do anything to the default XML configuration provided by Tomcat upstream, and we don't have any plans to do so.

I would recommend petitioning Tomcat upstream to add support for specifying the port to listen on via an environment variable instead, or using exactly the type of workaround you've provided here.