How to copy from HOST to CONTAINER while within container

I am sorry for taking up your time.

I have a local docker setup and I want to copy files from my local host to my container.
But the thing is that I need a command that I can use WHILE i am inside the container.

To explain the situation further: I executed “docker exec -it CONTAINERNAME bash” to enter my container,
and now I am on /var/www/html
and I need to find a way to copy a file/folder from my local environment into that container.

Reason: I am currently writing a dockerfile which automates the process of setting things up. I need that very specific command because a Dockerfile RUN-command can only be executed while inside the container.

What I tried:

“docker cp” is a good command to use when I am outside the container but it doesn’t work while in the container.

“DOCKERFILE COPY” might do the trick but I need a general shell command to double check if it really does what it is supposed to do. I must be able to reproduce the same process of my Dockerfile via manually executing the commands one by one.

Once again, I apologize for my inability to solve this problem by myself. My inexperience has caused me nothing but trouble.

Edit: I am using a Win10 64bit OS with dual monitor setup and a lefthanded mouse. My keyboard, albeit old, should possess all the necessary keys to replicate any essential keyboard-shortcuts if required. All my drivers are installed and updated.

Source: StackOverflow