docker "cp foo.txt mycontainer:/PATH/" does not work

Dear people,

I have started working with Docker today.

Not primarily to learn Docker, but I have an application to install, elche is pre-installed under Docker. I find this practical and convenient.

So far everything works fine. But somehow I misunderstand the Docker documentation. I want to copy a file from the host to the Docker instance.

I have a Docker instance running:

$ docker ps
CONTAINER ID IMAGE COMMAND     CREATED STATUS         PORTS NAMES
54e99d73543f od    "/bin/bash" 2 hours ago Up 2 hours       compassionate_bartik

This syntax "docker cp foo.txt mycontainer:/" does not work.

The command

docker cp test.txt compassionate_bartik:/ 

is executed, no error message is displayed, but no file is copied.

Only this construction works:

tar -cv *.txt | docker exec -i compassionate_bartik tar x -C .

What did I get wrong with the command "docker cp"?

How do you copy or export data into the docker container? Or don't you do that?

Best regards

Joern