Chore: Refactor Docker stuff

[zonescape]

This PR adds some improvements to Docker stuff. The main goal is to run phantun under unprivileged user and to use the default bridge network.

docker-compose.yml

• drop version field. It isn't used by modern Docker.
• drop fields that are unnecessary for the example (container_name, restart)
• don't use host network
• don't run in privileged mode
• change port numbers to those used in README
• use lowercase RUST_LOG value
• add UDP echo service as an example backend. This is very convenient for testing.

Dockerfile

• don't use latest as base image version
• change base image for runtime from debian to debian-slim to reduce image size
• drop dead code
• add net_admin capability to phantun executables to be run by unprivileged user
• move package installation before copying executables for better Docker build cache usage
• add unprivileged user
• use lowercase RUST_LOG value

.dockerignore

• change file name to Dockerfile.dockerignore. Current .dockerignore file isn't used by Docker.
• fix file patterns

phantun.sh

• move sysctls to docker-compose.yml. This is the usual place for them. Setting them in the entrypoint requires additional capabilities.
• run phantun under unprivileged user

Notes:

• I test image and compose file in the IPv4 mode
• I didn't test them in the IPv6 mode