Install Poetry stable when v1 is released

Following #149 (closed) and #245 (closed), switch back to Poetry stable:

• in Docker images: docker/python3/poetry@ac8190c6
• in Poetry installation documentation

changed milestone to %0.8.0

added CI/CD Dev Env Documentation Easy pick labels

changed title from Install Poetry stable when v1 released to Install Poetry stable when v1 is released

assigned to @moul

marked the checklist item in Docker images as completed

mentioned in commit 06580c73

changed the description

marked the checklist item in Poetry installation documentation as completed

closed

added Dep label