Docker orchestrator sometimes goes above limit

[TilBlechschmidt]

🐛 Bug description

When creating a large number of sessions (e.g. 30) and closely observing the docker orchestrator, it sometimes deploys exactly twice the amount of containers it is supposed to create.

🦶 Reproduction steps

Steps to reproduce the behavior:

1. make install
2. cargo run -- http://127.0.0.1:8080/ 35 firefox (with this)
3. Observe number of containers closely
4. See that sometimes 10 instead of 5 containers are started

🎯 Expected behaviour

The orchestrator should never go above its limit.