fix error with unix socket not resolving on dial

[faide]

the first example of README fails with:

panic: Get http://unix.sock/images/json?: dial tcp: lookup unix.sock on 127.0.1.1:53: no such host

on a Ubuntu.

After investigations it appears the Dial attribute of the hashicorp cleanhttp.DefaultTransport() is deprecated. Using the new recommanded DialContext fixes the issue...

[fsouza]

@faide that's surprising. The example on README should work fine. Can you provide more information on your environment? Is Docker bound to /var/run/docker.sock and does your user has access to it?

[faide]

Hello,

I created a sample project that reproduces the issue:

https://github.com/faide/dockerpilot

In this project I used govendor to pin dependencies that were used at the time. Nothing fancy here I just pulled the latest of each dependency and pinned the versions with govendor to help reproduce.

My system is:

• Ubuntu 16.04 LTS
• Go 1.7.4 (go1.7.4 linux/amd64)
• docker version : Docker version 1.13.0, build 49bf474
• my user is allowed to use the Docker daemon through CLI (I am part of the docker group, the command groups shows it)
• the docker unix socket exists and is writable for the docker group: (srw-rw---- 1 root docker 0 févr. 12 17:04 /var/run/docker.sock)

[faide]

to reproduce:

install govendor: go get -u github.com/kardianos/govendor

checkout master of the demo project (Without the PR applied to the go-dockerclient), govendor sync to get the correct vendor tree then build and run: you get an error:
panic: Get http://unix.sock/images/json?: dial tcp: lookup unix.sock on 127.0.1.1:53: no such host

checkout the pr-activated branch of the demo project (the current PR is applied to the vendored go-dockerclient) govendor sync to get the correct vendor tree and rebuild the demo project... it works and can enumerate the images

[faide]

@fsouza this commit is the source of this issue: hashicorp/go-cleanhttp@a459706

as you can see the latest go-cleanhttp is not using transport.Dial anymore but transport.DialContext, thus the code was changing the false Dialer and was having no effect if you were using the latest go-cleanhttp revision.

I suspect this is the case only with go-1.7+ (as go-1.6 and lower are just broken by the latest go-cleanhttp revision and cannot work anymore)

This commit is from feb 11, I must have stumbled on it just a few minutes after it was committed :)

[faide]

@fsouza btw this means that all test in the matrix that are using go < 1.7 will fail with the newer version of hashicorp cleanhttp. According to this you can just drop support for go 1.6

I do not get why I have a failure on 1.7 with the i386 build with docker 1.11.2. This appears only on i386 on this specific version of docker... strange.

[kcmerrill]

@faide thanks for that PR! I was wondering what was going on ...

@fsouza not sure if this helps in the troubleshooting, but master works when the unix socket is local, but it didn't work when I was using the unix socket inside a container(golang:1.7). I had everything vendored too. Hopefully that's not a red herring but it's a problem I observed.

[ewoutp]

This fix works fine with docker on Mac.

[fsouza]

I see what's going on, this is also related to #623.

I'm probably going to drop Go 1.6 and merge this PR. @faide how urgent is this for you at the moment?

Thanks for contributing!

[fsouza]

Quick note: I decided to merge this after reading the reports on #622.