How to write files in Docker Images? - docker

Question

I've copied a file into a docker image with:
COPY dbconfig.xml /var/app/dbconfig.xml
After that I tried to replace some values in the file with:
RUN sed -i "s/PASSWD/$dbpasswd/" /var/app/dbconfig.xml
Note that $dbpassword is an ENV Variable.
When I check the contents of config.xml, by starting a container of that image and running a bash inside it, nothing has changed in the dbconfig.xml.
Now I think I misunderstand some fundamentals of docker images..
I even tested to create a simple file:
RUN echo "test" > newfile.txt
which seems to be deleted after the call..
I know that each RUN statement creates an new layer and after the statement it gets removed(?).
I'm confused. Why does something like installing software with
RUN apt-get install -y some-package
doesn't get removed and creating a simple file does get removed?
So.. how can I change files inside docker images at image-build-time?
Dockerfile:
FROM dchevell/jira-software:8.0
COPY dbconfig.xml /var/atlassian/application-data/jira/dbconfig.xml
WORKDIR /var/atlassian/application-data/jira
# set default password to admin
ENV dbpasswd=admin
RUN sed -i "s/PASSWD/$dbpasswd/" dbconfig.xml \
&& cat dbconfig.xml
RUN echo "test" > newfile.txt
dbconfig.xml
<?xml version="1.0" encoding="UTF-8"?>
<jira-database-config>
<name>defaultDS</name>
<delegator-name>default</delegator-name>
<database-type>postgres72</database-type>
<schema-name>public</schema-name>
<jdbc-datasource>
<url>jdbc:postgresql://docker-postgres:5432/jiradb</url>
<driver-class>org.postgresql.Driver</driver-class>
<username>atlasdb</username>
<password>PASSWD</password>
<pool-test-while-idle>true</pool-test-while-idle>
</jdbc-datasource>
</jira-database-config>
Update 1
Confusingly, when I COPY something in the WORKDIR folder, it persists, but when I try to modify it afterwards with SED, these changes do not persist! I think there is some really dark magic happening in the background..
Maybe I try to bind mount my preconfigured dbconfig.xml within docker-compose and see if that helps..
Update 2
From the Docker Documentation:
Changing the volume from within the Dockerfile: If any build steps
change the data within the volume after it has been declared, those
changes will be discarded.
I totally missed that! Thanks David for pointing me there:) So creating and writing Files DOES work as expected, but be careful with VOLUME directories. RUN statements do not work here.
So to address this issue, the best practice would be to bind mount the file into that volume.

Answer 1

If you look at the Dockerfile for that base image, it says in part
ENV JIRA_HOME /var/atlassian/application-data/jira
VOLUME ["${JIRA_HOME}"]
Once you execute a VOLUME statement in a Dockerfile, later Dockerfile statements can't make any more changes in that specific directory.
Given that the sorts of things you're trying to change are very installation-specific settings (admin password, database settings) I wouldn't try to build an image out of these. Instead I'd use the docker run -v option to inject the configuration file at runtime.

Answer 2

Each RUN statement does not create an intermediate container but creates a new layer on union file system, which is read only. When you run an image, a special writable layer is created for this container and all the changes you make on this container are written to this layer. (except the volumes. which is a different concept). That is why docker is able to share the same image (or even layers) between containers safely, without affecting each other. You can check docker documentation for more information.
For your question, you should see every change you make on build time in the running instance of this image, unless you somehow delete or overwrite them.

Answer 3

See this question.
The commands you are running are correct and they should create the files. What I suspect is that when you run your container, the jira application is overwriting the WORKDIR you have specified.
Try this Dockerfile:
WORKDIR /var/atlassian/application-data/jira
# set default password to admin
ENV dbpasswd=admin
RUN sed -i "s/PASSWD/$dbpasswd/" dbconfig.xml \
&& cat dbconfig.xml
WORKDIR /testtest
RUN touch test.txt
RUN echo "test" > newfile.txt
WORKDIR /var/atlassian/application-data/jira
Now if you start the container, you can see that the files are being created inside the /testtest folder.
If you want your changes to the dbconfig.xml file to persist you should try using volumes to bind the local dbconfig.xml with the jira folder.
Thanks for this interesting question :)