I haven't used it, but step-ca might be a good choice:
Edit:
To expand, there are some past threads discussing using Boulder for an internal CA. For example, this one:
The Boulder developers don't recommend it, since it's not designed to be easy to install, or easy to use in an environment less complicated than a full-on publicly-trusted CA.
It might be a worthwhile project, depending on your goals, but it's not the easiest way to run an internal CA.
Also, I forgot to mention it, but EJBCA Enterprise also supports ACME.