4.1.2.3 Entrypoint Documentation
A focused guide to Entrypoint Documentation, connecting core concepts with practical Docker and container operations.
Entrypoint documentation is the practice of clarifying, through comments or accompanying documentation, exactly how a container's ENTRYPOINT is meant to be used — what arguments it accepts, what behavior changes are possible, and what the default execution path actually does — since this is often less self-evident than other parts of a Dockerfile.
Why Entrypoint Behavior Can Be Non-Obvious
Unlike CMD, which can simply be replaced entirely, ENTRYPOINT defines a fixed executable that arguments are appended to, which can be confusing for someone unfamiliar with this distinction trying to figure out how to pass custom options to a container.
ENTRYPOINT ["myapp"]
CMD ["--mode", "production"]
docker run myimage --mode development
Without documentation, it might not be obvious that this command results in myapp --mode development, rather than replacing the entire startup command.
Documenting an Entrypoint Script's Behavior
When the entrypoint is a custom shell script performing setup logic before launching the actual application, documenting what that script does is particularly valuable, since its behavior is not visible just from reading the Dockerfile itself.
COPY entrypoint.sh /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]
#!/bin/sh
# entrypoint.sh
# Waits for the database to become reachable before starting the app.
set -e
until pg_isready -h "$DB_HOST"; do sleep 1; done
exec "$@"
A comment explaining that this script waits for database readiness before launching the application clarifies behavior that would otherwise require reading the script itself to discover.
Documenting How to Override Entrypoint Behavior
For debugging, it is often useful to document how the entrypoint can be bypassed entirely, which is occasionally necessary when troubleshooting a container that otherwise fails before a shell could be reached.
docker run -it --entrypoint sh myimage
Why Entrypoint Documentation Matters
Because ENTRYPOINT behavior is one of the more frequently misunderstood aspects of a Dockerfile, clear documentation around how it is meant to be used — particularly when custom scripts are involved — saves significant time for anyone trying to run, debug, or modify the container's startup behavior.