4.1.2.1 Runtime Requirement Docs
A focused guide to Runtime Requirement Docs, connecting core concepts with practical Docker and container operations.
Runtime requirement docs, in the context of a Dockerfile, are the parts of the file — particularly ENV, EXPOSE, and any comments describing expected external dependencies — that communicate what a container needs from its environment in order to actually function correctly once running.
Documenting Expected Environment Variables
While ENV sets default values, it also implicitly documents which environment variables the application is aware of and may expect to be configured, which is useful information for anyone deploying the image.
ENV DATABASE_URL=postgres://localhost:5432/app
ENV LOG_LEVEL=info
Even though these defaults might not be suitable for production, their presence in the Dockerfile reveals that the application reads these specific environment variables.
Documenting Expected Ports
EXPOSE documents which ports the application listens on, which is useful information for anyone configuring port mappings or network policies around the container, even though it does not enforce or guarantee network reachability on its own.
EXPOSE 8080
EXPOSE 9090
This reveals that the application uses two distinct ports, perhaps one for its main service and another for metrics or health checks.
Documenting External Service Dependencies Through Comments
Requirements that cannot be expressed through a Dockerfile instruction directly — such as needing access to a specific external database or message queue — are often documented through comments, since the Dockerfile syntax itself has no dedicated mechanism for declaring this kind of dependency.
# Requires network access to a PostgreSQL instance at DATABASE_URL
# and a Redis instance at REDIS_URL for session storage.
ENV DATABASE_URL=""
ENV REDIS_URL=""
Why Runtime Requirement Documentation Matters
Clearly surfacing what a container expects from its environment — required configuration, expected ports, external dependencies — directly within or alongside the Dockerfile reduces the chance that someone deploying the image discovers a missing requirement only after the container fails to start correctly.
docker run myapp
docker logs myapp
Without clear documentation of required configuration, a missing requirement often only becomes apparent through a runtime failure and its logs, rather than being anticipated ahead of time.