docker: install ansible-galaxy contents if desired

[commx]

This PR adds the ability to install ansible-galaxy collections and roles on Semaphore startup when using the Docker image, so collections and/or roles don't need to be installed every time a task is launched.

The approach is very similar to what can be accomplished with the "pre-commands" in the systemd service as described in the manual installation section of the administration guide.

Assuming you have the following requirements.yml:

---
collections:
  - name: ansible.posix
    version: "1.5.4"

roles:
  - name: geeringguy.node_exporter
    version: "2.1.0"

Mount it to the Semaphore container with the docker compose service entry:

---
services:
  semaphore:
    image: semaphoreui/semaphore:latest
    environment: {}
    volumes:
      - path/to/requirements.yml:/etc/semaphore/requirements.yml:ro