It depends on your employer. For example, with Oak Ridge National Laboratory, if you are employed by them, you need to get permission from your boss before you can work elsewhere. I think if you are in a classified job, or a job where there can be ethical conflicts, such as in a role where you make purchasing decisions, then you may find that the business that can supply to your company may be a problem.
But, if you don't have a policy in the corporate handbook restricting you then you should be free to do on your time what you want.
This also may be a problem if your company has first dibs on any IP that you do, even if on your own time, with your own equipment.
So, look carefully at the employee handbook then see if any of this applies, otherwise, start one up and good luck.