Ease use in (azure) cloud environment

[jpicht]

We faced the challenge to forward the containerd container logs from inside of an azure managed kubernetes cluster to papertrail. We tried fluentd and weren't very happy with the results. This is what we came up with.

Changes:

• Config option for a cluster domain, that is suffixed to the host name.
• Optional regular expression per glob pattern, to extract a meaningful "tag" from the file name.
• Example Dockerfile and kubernetes manifest
• Updated README