Ansible - Copy multiple files

I am trying to copy multiple files to the target like so:

tasks:
  - name: "Copying files"
    copy:
      src: files/{{ item }}
      dest: /my/folder/
    with_items:
      - file.txt

The files directory is in the same directory as the Ansible playbook. However when running the script, I get this error:

docker: TASK [Gathering Facts] *********************************************************
    docker: fatal: [test]: FAILED! => {"msg": "failed to transfer file to /home/user/.ansible/tmp/ansible-local-123331or28t2a/tmp6st_fxv1 ~user/.ansible/tmp/ansible-tmp-1579990845.0734024-189911264147319/AnsiballZ_setup.py:\n\n\n"}
    docker:

Why is that? Why can Ansible not copy the files to the target machine?

Following you comment: no I cannot reproduce your problem. Since you did not provide the rest of your files (inventory, etc...) and I cannot guess where is your problem, here is a quick proof by example using the php:7.0-apache image.

• The inventory (inventories/docker_test/hosts.yml)

---
all:
  hosts:
    docker_test:
      ansible_connection: docker

• Launching the test container

docker run -d --rm --name docker_test php:7.0-apache

• The test playbook.yml

---
- hosts: docker_test
  gather_facts: false

  tasks:
    # Your image does not have python which is mandatory for ansible
    # Please note this is for demo only. You should get python installed
    # by extending the image through a Dockerfile and build it.
    - name: Dirty low-level command to get python installed
      raw: apt-get update && apt-get install -y python3

    - name: Gather facts now we can
      setup:

    - name: Copy a dummy file
      copy:
        dest: /etc/apache2/sites-available/dummy.conf
        content: "# I'm a dummy file"

• Playbook run and output

$ ansible-playbook -i inventories/docker_test/ playbook.yml 

PLAY [docker_test] ********************************************************************************************************************************************************************************************************

TASK [Dirty low-level command to get python installed] ********************************************************************************************************************************************************************
changed: [docker_test]

TASK [Gather facts now we can] ********************************************************************************************************************************************************************************************
[WARNING]: Platform linux on host docker_test is using the discovered Python interpreter at /usr/bin/python3.5, but future installation of another Python interpreter could change this. See
https://docs.ansible.com/ansible/2.9/reference_appendices/interpreter_discovery.html for more information.

ok: [docker_test]

TASK [Copy a dummy file] **************************************************************************************************************************************************************************************************
changed: [docker_test]

PLAY RECAP ****************************************************************************************************************************************************************************************************************
docker_test                : ok=3    changed=2    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

StackExchange DevOps Q#10597, answer score: 1