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Issue Type: Bug

I'm behind a company proxy so I usually need to add these settings when installing something for VScode.
When trying to install an extension in my container by using the .devcontainer.json file

{
    "dockerFile": "Dockerfile",
    "extensions": [
        "ms-vscode.cpptools"
    ],
    "runArgs": [
        "--cap-add=SYS_PTRACE",
        "--security-opt",
        "seccomp=unconfined"
    ],
    "settings": {
        "http.proxyStrictSSL": false,
        "http.proxy": "http://myproxy.com:81"
    }
}

Fails to actually use these settings or my own vscode settings to install through the proxy. From what I can tell these settings only apply within the container to install anything supplementary used by the extension (ie C++ needs to install 4 extra things). Weirdly if I try to install via the UI it works with no issue.
I think the codepath to install extensions within the container isn't respecting any proxy settings set in VSCode itself.
From the trace it also looks like it tries to install the extension via an IP Address instead of a url which is also explicitly blocked on our end.

When adding exports to my container

ENV http_proxy=http://myproxy.com:81 \
       https_proxy=http://myproxy.com:81

I get an error self signed certificate in certificate chain :-(

Extension version: 0.66.0
VS Code version: Code 1.36.1 (2213894ea0415ee8c85c5eea0d0ff81ecc191529, 2019-07-08T22:56:38.504Z)
OS version: Darwin x64 18.6.0
Remote OS version: Linux x64 4.9.125-linuxkit