unless you work 24/7 your own time should still be yours. however, anything generally developed / worked on on company resources is theirs. at least that how it works for my contract. That means u can on your own machines and own time make something for yourself. but if you ever once accidentally even load it onto work computer, it belongs to them.

if they don't even let you do that, then i would firstly seek a job which does let you work for yourself on your own time, and then start really pushing for your own company. Learning how to do so is free for you to pursue, as long as you don't 'make' anything on the companies' system / resources it should be out of their sight and safe to do.