Imagine in your stack you have RESTful services which also provide some rudimentary frontends, mostly for admin/other tech user use.
Do you include the UI inside the container or are these two containers? Why?
Are they separate projects (from a logical or technical perspective)? Would one ever need to be updated/recreated independently of the other one? Would they need to scale independently of each other? I think if any of those answers is a yes, then they should be separated. If not, you should be fine bundling them together. Bundling them together will reduce discrepancy or compatibility issues between the two containers.
We always use different container for each service (Like Frontend ,Backend and Database). Because a container can run only a single service inside it at a time by using docker-compose.yml file. But we can define multiple services inside one docker-compose.yml file but one for each container.