Optimise Docker image

[adintegra]

We should further optimise the Docker image both in terms of size and interoperability. Specifically, we should:

Incorporate multi-stage build to optimise image size
This puts less strain on system resources (i.e. large file transfers) and should make deployments a little faster. Obviously, it also reduces storage requirements.

Properly handle system signals (e.g. SIGTERM)
This ensures so the container can be shutdown gracefully by the host environment and minimizes the risk of data loss or corruption.

Not run node as root
This helps to better secure the container.