[DO NOT MERGE] build: add rc.3 docker

[jnweiger]

[phil-davis]

tag: v10.15.0-rc.3 does not have the commit for "[10.15.0] chore(deps): bump phpseclib/phpseclib from 3.0.38 to 3.0.39 (#41276)" in it.

[phil-davis]

We need updated PHP dependencies for files_external_dropbox https://github.com/owncloud/files_external_dropbox/pull/161

To fix the Trivy messages about guzzlehttp/psr7

[phil-davis]

And similar for: https://github.com/owncloud/openidconnect/pull/314

I think that releases will need to be done for those 2 apps, so that a version of them with up-to-date dependencies can be bundled with 10.15.0

[jnweiger]

Thanks @phil-davis for those updates,

• the tag for rc.3 was off by one. ( I keep doing this all the time :see_no_evil: )
• will make releases for dropbox and openidconnect
• then we'll call it RC 4

[phil-davis]

@jnweiger this is out-of-date since PR #488

close?