Add Support for communication to docker hosts using UNIX domain sockets

[twz123]

This is my draft to a solution for #148. The same technique is used by the Spotify plugin. No obscure packaging involved.

• Use jnr-unixsocket for native OS-level access
• Use a pseudo socket class that mimics a Java network socket and delegates to the jnr channel where applicable
• Provide a special ApacheUnixSocketHttpDelegate that enables the Apache HTTP client to use that pseudo socket for HTTP communication

[rhuss]

Awesome! Thanks a lot. Do you know any limitations of jnr-unixsocket ? (i.e. does it run on any Unix or are there are limitations ? Just to add it to the docs in case).

BTW, whould you mind to signoff the PR ? (as explained here). Shouldn't be much more than a git commit -s --amend on this branch. Thanks !

I will review and integrate it ASAP, probably this week.

[twz123]

No, I don't have any deeper knowledge of jnr-unixsocket. The only two things I can state about it:

• Spotify uses it in their docker plugin
• It simply just worked on first shot when implementing this PR

Added the signoff.

[rhuss]

Thanks ! Give me some days for the review, won't make it this week but hopefully the week after.

Thanks again ...

[rhuss]

Merged it into the integration with some slight modifications:

• Detangled inheritance hierarchy
• Avoid encoding the socket path as an URL host name. Instead set the path directly in the constructor of UnixSocketFactory. The base URL (host & port) is now meaningless and set to a dummy value.
• If no DOCKER_HOST is set or configured, and a file /var/run/docker.sock exists, takes this as a default.

Again, thanks a lot ! Especially for finding out how Apache HC works nicely with a Unix Socket.

The next release 0.12.0 containing the changes will come next week.