Add support for scripts during initialization

[vk496]

This functionality was suggested by #387 (comment)
I saw @Caian PR #6577 after, so I publish my suggestion anyway

This patch will enable more complex init patterns during the IPFS first initialization. The scripts under /docker-prehook.d directory will be executed just after all env initialization and BEFORE running ipfs daemon. If there were no errors, it will execute the ipfs daemon. AFTER running ipfs daemon (10 seconds) it will execute all scripts under /docker-posthook.d and kill the main ipfs process if any script failed.

Btw, I'm not sure if the documentation of this functionality should go to the README.md or to https://github.com/ipfs/ipfs-docs/

[BigLep]

@olizilla or @gmas : does the help from an infra regard? We wanted to check to see if this would be useful.

Also, is this approach preferred over #8326 ?

[lidel]

Thank you @vk496 for poking at this!

If we remove posthook due to sleep 10 (see comment inline), this PR feels redundant.
Let's continue in @Caian's PR #6577 instead.

[lidel]

Separate stage after sleeping for 10 seconds feels brittle. Could not be enough if my node runs on slow CI and takes longer time to init/start etc.

I feel we don't need a separate directory with posthooks executed after a hardcoded delay. If anything, we would be waiting for node to start (eg. by probing /api/v0/id on the API port etc).