I can not do the stack file docker-cloud.yml command that appends text data to the end of the file

What we have: a stekfayl on dokerklaud, which tightens the image and runs the container. What you need: with the help of the command, add text information to a file, which, for example, lies in the / root / aws / testfile directory as with the help of the >> operator to add text information (in this case, keys) to a file in the system. The file can not be attached to the assembly stage of the dokerfile. What I tried: I tried enripoint, I tried the command: I tried this for example: command: "echo -e \" aws_access_key_id = A * I ******* HC3 ****** \ naws_secret_access_key = ***** *** T ******* O + ****** U *** SM **** 3 *** d \ ">> /root/.aws/credentials" in fact, what I am have tried