Support proxy environment for docker client

[PMExtra]

Proxy environment works fine with docker-cli:

> export http_proxy=http://my_proxy:8080
> export DOCKER_HOST=tcp://my_docker_host_through_proxy
> docker info
# Works fine
> docker image inspect foo:bar
# Works fine

But does not work with skopeo:

> export http_proxy=http://my_proxy:8080
> skopeo inspect --daemon-host http://my_docker_host_through_proxy docker-daemon:foo:bar
FATA[0000] Error parsing image name "docker-daemon:foo:bar": initializing docker engine client: unable to parse docker host `my_docker_host_through_proxy`

It's just an example, I need to copy image to the docker-daemon host in actual.

After this pull request was patched, skopeo also worked fine with the proxy environment.

Note that: I used a plain HTTP connection with docker-daemon, so it should be http:// in skepeo but tcp:// in docker-cli.

[mtrmac]

Thanks!

After this pull request was patched, skopeo also worked fine with the proxy environment.

This does not even compile, so I can’t see how this could have possibly worked.

---

Also, per https://github.com/containers/image/blob/main/CONTRIBUTING.md#sign-your-prs :

Use your real name (sorry, no pseudonyms or anonymous contributions.)

[PMExtra]

@mtrmac I've fixed it. Please let me know if there is anything else I need to do.

[rhatdan]

LGTM

[mtrmac]

If this makes sense doing, then the http: and https: protocols should support proxies equally.

(Alternatively, AFAICS we don’t actually need to configure this manually: comparing https://github.com/docker/cli/blob/57a1180c52ad7633dcf2fe1515ec2601ffb01944/cli/context/docker/load.go#L85 and looking at the implementation, if WithHttpClient is included before WithHost, that triggers sockets.ConfigureTransport and that sets this, along, with a few other options. But that’s undocumented and very nonobvious; it’s really only a confirmation that ProxyFromEnvironment can be appropriate here.)

[rhatdan]

@PMExtra still working on this?

[PMExtra]

I don't know what I should do. I think it's ready for merging.

[rhatdan]

@mtrmac what would you like to see done?

[mtrmac]

If this makes sense doing, then the http: and https: protocols should support proxies equally.

This only adds them to the former; add support for the latter.

[PMExtra]

@mtrmac I'm not sure what you mean. http.ProxyFromEnvironment can support several variables such as HTTP_PROXY, HTTPS_PROXY, and NO_PROXY.

If you set HTTPS_PROXY, it will work with https:.

[PMExtra]

@mtrmac see https://pkg.go.dev/net/http#ProxyFromEnvironment

[mtrmac]

The httpConfig function is not used for https, so whether http.ProxyFromEnvironment would have worked for https is irrelevant when this PR is not invoking it in that case.

[mtrmac]

That example does not reference the httpConfig function (and, more to the point, its caller) at all, and is irrelevant to my objection.

[PMExtra]

@mtrmac Now I finally understand what you meant, and I apologize for the previous misunderstanding.

[mtrmac]

Thanks!

Code LGTM, please squash into one commit and rebase.

[mtrmac]

Thanks!