Move a few shared classes and constants to new util.shared module

Now that GitHub has a copy button for each code block it's far too easy to copy/paste a whole bunch of sample output

Also removed % from start of lines so that sample commands will run without editing.

Interval is no more limited to "seconds":

```shell
./dnsping.py -i 0.2 -c 50 --flags  -t AAAA -s 9.9.9.9 ripe.net
```

Fixes #89

* Change image to python:3.8-alpine targeting a smaller image (50mb now)
* Added /dnsdiag to path, allowing path removal from execution
* Change on README to reflect path change and recommending network mode
  as host, since it will reflect a better scenario from host.

- Updated dependencies (required for new features)
- Only Python 3.7 and above are supported

Add support for "-m" / "--cache-miss" flag to prepend fqdn in request
with a random 8-bytes string to enforce a cache miss.

This will help users to measure delay the whole recursion chain, by
enforcing DNS resolver to contact "resolve" the name every time.
As a consequence, the response may be empty or contain a wildcard
response.

Add support to clear RD flags from requests to disable recursion,
using "-r" or "--norecurse" flag.