Support HTTP_PROXY / HTTPS_PROXY / NO_PROXY for connecting to portainer endpoint.

[skokado]

Is your feature request related to a problem? Please describe

The provider builds its http.Transport manually (for skip_ssl_verify), which
leaves Proxy unset. Unlike http.DefaultTransport, it ignores the standard
HTTP_PROXY / HTTPS_PROXY / NO_PROXY environment variables and always dials
the Portainer endpoint directly. There is currently no way to use the provider
when Portainer is only reachable through an HTTP(S) proxy.

Describe the solution you'd like

Add Proxy: http.ProxyFromEnvironment to the transport — the same behavior as
http.DefaultTransport. Proxy env vars are honored when set, and it is a no-op
when unset, so existing users are unaffected. I have a patch ready and can
submit a PR.

Describe alternatives you've considered

• A proxy_url provider attribute: also viable, but env vars are the standard
  Go convention and need no schema change.
• A local TCP forwarder (socat/gost) in front of the endpoint: works, but adds
  extra tooling for what Go supports natively.

Additional context

No response

[skokado]

PR is ready (waiting for reply)
skokado#1

[grulicht]

Hi @skokado,
thanks a lot for using the Portainer Terraform provider and for taking the time to report this issue – your detailed reproduction and example repo are super helpful! 🙌

We're investigating the root cause and will include a fix in one of the upcoming releases. We'll make sure to notify you once it's available. 🚀

[grulicht]

Hi @skokado ,
New release version v1.34.0 with update is out! 🙌🏽