ci.yml: Trigger docker image creation only on workflow_dispatch

[tgonzalezorlandoarm]

• Simplify the docker creation process to avoid having to push a new commit with changes to the codebase.
• Now it will only be necessary to trigger the CI workflow manually with the input 'parsec-service-test-all'.

[tgonzalezorlandoarm]

Manual run: https://github.com/tgonzalezorlandoarm/parsec/actions/runs/6865120749