Recommended way to handle empty vs existing DB in Dockerfile - docker

Question

I want to run M/Monit (https://mmonit.com/) in a docker container and found this Dockerfile: https://github.com/mlebee/docker-mmonit/blob/master/Dockerfile
I'm using it with a simple docker-compose.yml in my test environment:
version: '3'
services:
mmonit:
build: .
ports:
- "8080:8080"
#volumes:
#- ./db/:/opt/mmonit/db/
It does work, but I want to extend the Dockerfile so that the path /opt/mmonit/db/ is exported as a volume. I'm struggling to implement the following behaviour:
When the volume mapped to /opt/mmonit/db/ is empty (for example on first setup) the files from the install archive should be written to the volume. The db folder is part of the archive.
When the database file /opt/mmonit/db/mmonit.db already exists in the volume, it should not be overwritten in any circumstances.
I do have an idea how to script the required operations / checks in bash, but I'm not even sure if it would be better to replace the ENTRYPOINT with a custom start script or if it should be done by modifying the Dockerfile only.
That's why I ask for the recommended way.

Answer 1

In general the strategy you lay out is the correct path; it's essentially what the standard Docker Hub database images do.
The image you link to is a community image, so you shouldn't feel particularly bound to that image's decisions. Given the lack of any sort of license file in the GitHub repository you may not be able to copy it as-is, but it's also not especially complex.
Docker supports two "halves" of the command to run, the ENTRYPOINT and CMD. CMD is easy to provide on the Docker command line, and if you have both, Docker combines them together into a single command. So a very typical pattern is to put the actual command to run (mmmonit -i) as the CMD, and have the ENTRYPOINT be a wrapper script that does the required setup and then exec "$#".
#!/bin/sh
# I am the Docker entrypoint script
# Create the database, but only if it does not already exist:
if ! test -f /opt/mmonit/db/mmonit.db; then
cp -a /opt/monnit/db_base /opt/monnit/db
fi
# Replace this script with the CMD
exec "$#"
In your Dockerfile, then, you'd specify both the CMD and ENTRYPOINT:
# ... do all of the installation ...
# Make a backup copy of the preinstalled data
RUN cp -a db db_base
# Install the custom entrypoint script
COPY entrypoint.sh /opt/monit/bin
RUN chmod +x entrypoint.sh
# Standard runtime metadata
USER monit
EXPOSE 8080
# Important: this must use JSON-array syntax
ENTRYPOINT ["/opt/monit/bin/entrypoint.sh"]
# Can be either JSON-array or bare-string syntax
CMD /opt/monit/bin/mmonit -i
I would definitely make these kind of changes in a Dockerfile, either starting FROM that community image or building your own.