Put "docker-entrypoint.sh" into PATH and allow it to run Cassandra when given no arguments

[tianon]

This also includes our standard backwards compatibility symlink which will automatically be removed for 3.12 and above.

[tianon]

$ ./update.sh
2.1: 2.1.19
2.2: 2.2.11
3.0: 3.0.15
3.11: 3.11.1

[CSdread]

This is for retrospective for you guys, we use 2.2.11 as our upstream container for a custom Cassandra cluster. This PR broke our cluster due to the fact that we have a custom docker-entrypoint.sh that we loaded into /. We have come to expect that a docker image with a specific tag will have stability which allows us to build on top of it. We would propose a build version on the image tag to prevent retro-active updating of a docker images affecting production systems.

@yosifkit @tianon

[tianon]

@CSdread sorry to hear this caused issues 😞

Honestly though, what I'd recommend in that situation is that instead of COPYing over the top of our file and expecting ENTRYPOINT to stay consistent, simply copy your file elsewhere (since you've already paid the storage cost for our script, however minimal) and use ENTRYPOINT in your sub-Dockerfile to point it at your script instead. That also allows you do do things like start up a shell from your descendant image and use diff to compare your entrypoint script to the image-provided one, and allows you to pipe your entrypoint to ours if necessary if you want both behaviors chained together (ie, set something like ENTRYPOINT ["/custom-entrypoint.sh", "/docker-entrypoint.sh"]).