Feat/dockerfile

[dbrennand]

Summary

This PR adds support for running the librespeed-cli in a container.

Files added/modified:

• dockerfile
• README.md

Working example:

➜  speedtest-cli git:(feat/dockerfile) docker build -t librespeed-cli:latest .
➜  speedtest-cli git:(feat/dockerfile) docker run --rm --name librespeed-cli librespeed-cli:latest
Retrieving server list from https://librespeed.org/backend-servers/servers.php
Selecting the fastest server based on ping
Selected server: London, England (Clouvider) [lon.speedtest.clouvider.net]
Sponsored by: Clouvider @ https://www.clouvider.co.uk/
You're testing from:
Ping: 23 ms     Jitter: 4 ms
Download rate:  18.72 Mbps
Upload rate:    37.10 Mbps