Make it possible to supply your own http client

[rrva]

No description provided.

[vieux]

I don't see why not, but could you please tell us what you are planning to do ?

[rrva]

At https://github.com/rrva/docker-devdns I wanted to specify a timeout on all docker operations.
Figure this is only sensible to do on read requests, not on write requests (would not want to fire-and-forget a run container command in the same way), but I want to timeout pretty aggressively on ListContainers / InspectContainer requests. I use ResponseHeaderTimeout transport setting on the httpclient for this.

From http://golang.org/src/net/http/transport.go?s=1324:4038#L39

// ResponseHeaderTimeout, if non-zero, specifies the amount of
// time to wait for a server's response headers after fully
// writing the request (including its body, if any). This
// time does not include the time to read the response body.
ResponseHeaderTimeout time.Duration
// contains filtered or unexported fields