Is the inclusion of a regular language in a context-free language decidable?

This is a correct answer for $R \subseteq C$ for regular $R$ and context-free $C$. The question also mentions the reverse inclusion $C \subseteq R$.

Somewhat surprisingly, that is decidable. We have $C \subseteq R$ iff $C \cap R^c = \emptyset$. Here $R^c$ is the complement of $R$ is regular, and thus the intersection $C \cap R^c$ is context-free. Emptiness of context-free languages is decidable.

It is a well-known undecidable problem whether a context-free grammar generates all possible strings over the alphabet. Therefore your problem must also be undecidable, because it is easy to write down a regular grammar for all strings.