Add docker-ensure-installed.sh as an explicit entrypoint/symlink

[tianon]

This allows using the image as docker run --mount ... wordpress docker-ensure-installed.sh which will then just ensure WordPress is installed in the volume and exit (successfully).

The primary use case for this is an initContainer in Kubernetes.

• Closes Support running only the WordPress installation part from the entrypoint #969
• Closes Allow to run only the WordPress initialization sequence #970
• Closes Argument check in docker-entrypoint.sh #805

See also:

• Support running only the WordPress installation part from the entrypoint #969 (comment)
• https://kubernetes.io/docs/tasks/configure-pod-container/image-volumes/
• Implement mount from image moby/moby#48798
• Add new "docker-ensure-initdb.sh" script postgres#1150

[yosifkit]

This seems good to me but feels a bit reactionary (though it has been a fairly common ask in the past). Should we let it wait a bit to give users time to express their 👍 or just go for it?

Just to link a few past requests:

• docker-entrypoint.sh conditional check prevents extension of CMD #691
• Hook in to specific point in entrypoint script #147
• entrypoint on subsequent layers #130
• added optional entrypoint directory #124

[tianon]

One way to look at it is that all those requests have been sitting in the back of my head simmering such that the latest iteration of it felt like it was "finally time" 😅