Add Testcontainers User-Agent

[eddumelendez]

Add the following User-Agent tc-elixir/<version> when making calls to Docker Engine.

Looking at the code, I think the change should be added into the options here

[jarlah]

i might need to find out how to pass headers into the docker engine library. Either update the library or use existing ways to pass in headers. To be figured out.

[jarlah]

ui think we use same technique as in #90 if it works