Docker-CE: How to modify containers with overlays / How to add directories to a standard docker image

After some experiments with docker i wanted to run a tomcat with my own configuration (e.g. memory settings, ports, ...).

My first idea was: Download tomcat, configure everything and then build an image.
BUT: After i learned how to use the -v (--volume) flag for adding some file via the docker command to an image i was wondering, wether creating a new image with only the additional files on top of standard tomcat docker image.

So first step is to take a look at all local images:

# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
558MB
friendlyhello       latest              976ee2bb47bf        3 days ago          148MB
tomcat              latest              11df4b40749f        8 days ago          558MB

I can use tomcat:latest. (if it is not there just pull it: docker pull tomcat)
Next step is to create a directory and add all the directories which you want to override.
For my example:

mkdir conftomcat
cd conftomcat
mkdir bin

Into the bin directory i put all the files from the tomcat standard container:

# ls bin
bootstrap.jar  catalina-tasks.xml  commons-daemon-native.tar.gz  daemon.sh  setclasspath.sh  startup.sh       tool-wrapper.sh
catalina.sh    commons-daemon.jar  configtest.sh                 digest.sh  shutdown.sh      tomcat-juli.jar  version.sh

Inside the catalina.sh i added -Xmx384M.
In conftomcat i created the following Dockerfile:

FROM tomcat:latest
WORKDIR /usr/local/tomcat/bin
ADD status /usr/local/tomcat/webapps/mystatus
ADD bin /usr/local/tomcat/bin
ENTRYPOINT [ "/usr/local/tomcat/bin/catalina.sh" ]
CMD [ "run"]

And as you can see i added my index.jsp which is inside status (s. this posting).
Ok. Let's see, if my plan works:

#docker build  -t  mytomcat .

ending build context to Docker daemon  375.8kB
Step 1/6 : FROM tomcat:latest
 ---> 11df4b40749f
Step 2/6 : WORKDIR /usr/local/tomcat/bin
 ---> Using cache
 ---> 5696a9ab99cb
Step 3/6 : ADD status /usr/local/tomcat/webapps/mystatus
 ---> 1bceea5af515
Step 4/6 : ADD bin /usr/local/tomcat/bin
 ---> e8d3a386a7f0
Step 5/6 : ENTRYPOINT [ "/usr/local/tomcat/bin/catalina.sh" ]
 ---> Running in a04038032bb7
Removing intermediate container a04038032bb7
 ---> 4c8fda05df18
Step 6/6 : CMD [ "run"]
 ---> Running in cce378648e7a
Removing intermediate container cce378648e7a
 ---> 72ecfe2aa4a7
Successfully built 72ecfe2aa4a7
Successfully tagged mytomcat:latest

and then start:

docker run -p 4001:8080 mytomcat

Let's check the memory settings:

$ ps aux|grep java
root      2313 20.7  8.0 2418472 81236 ?       Ssl  19:51   0:02 /docker-java-home/jre/bin/java -Djava.util.logging.config.file=/usr/local/tomcat/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -Xmx394M -Djdk.tls.ephemeralDHKeySize=2048 -Djava.protocol.handler.pkgs=org.apache.catalina.webresources -classpath /usr/local/tomcat/bin/bootstrap.jar:/usr/local/tomcat/bin/tomcat-juli.jar -Dcatalina.base=/usr/local/tomcat -Dcatalina.home=/usr/local/tomcat -Djava.io.tmpdir=/usr/local/tomcat/temp org.apache.catalina.startup.Bootstrap start

Yes - changed to 384M.
And check the jsp:

Yippie!
As you can see, i have the standard tomcat running with an override inside the configuration to 384M. So it should be easy to add certificates, WARs, ... to such a standard container.


Related posts:

• Docker on Ubuntu 
• Running through a tutorial / investigating the processes
• Creating a small linux for my docker cluster (Alpine Linux) 
• How to build a docker swarm 
• Creating a minimal webserver container and deploying it to docker swarm
• Setting up a tomcat with docker  
• Running a swarm with multiple managers 
• Mysql inside a docker container - persistency with docker volumes 
• How to change the configuration of a standard docker container(e.g. changing tomcats memory settings)