What are you trying to prevent? You can't release anything (opensource or not) without risking someone stealing the idea without patenting.
No FOSS license will prevent that (quite the opposite, it encourages copying/modifications). Those licenses just prevent someone using your code commercially without releasing the source code again.
no, the patent office would find your publication, deem it Prior Art and not grant the patent. If it would miss it (some don't research very well), anyone can notify them to void the patent afterwards anytime.
IANAL, there are lawyers specialized on patents who'll reassure you for free/cheap (relatively, they are friggin expensive). It also depends on legislature. Countries that break/never agreed to the PCT will do what they please.
You could do some automated/scripted installation VM-image builder thingy and release that. Would probably also save some manual work for you. (bash script fetching install image & run qemu, autounattend.xml, etc. all nicely released on github.) And it'd be auditable.
check carefully what you signed. If you didn't sign anything saying otherwise, there's nothing to prevent you from doing it.
If there's something, you could still work around it (e.g. remove company secrets).
If the resulting product is provable better, then it's objectively not the same thing you did for your boss.
After checking all of this, your local FSF might give you free legal advice to get going (keep all notes/correspondence secure for later if anything comes up. It proves you tried to act responsibly).
Great, now find a project to apply it and collect your participation trophy. :-P