Add and document TLS configuration for docker_logs source

[juchiast]

Documentation should mention how env var DOCKER_CERT_PATH and DOCKER_CONFIG is being used to get certificates when connecting via TLS.

In the same spirit as #5413, we should add TLS configuration options for docker_logs source.

Three required options:

• tls.ca_file
• tls.crt_file
• tls.key_file

Official docker client allows some of the options to be missing, but bollard does not, therefore all three options are required.