Lack of proxy support for snap prepare-image

[Soar Huang]

Hi there,

I've set proxy through "snap set system proxy.https" and then "snap install" works fine, but "snap prepare-image" doesn't work.
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(Ubuntu 18.04.2 LTS behind proxy)
$ snap --version
snap 2.38.1
snapd 2.38.1
series 16
ubuntu 18.04
kernel 4.18.0-17-generic

$snap prepare-image ....
error: Get https://api.snapcraft.io/api/v1/snaps/assertions/snap-revision/lUQCdOYc7CNWpZGJ6LrsKNj8CLE7CwE0PUHtCfXxYxtQo3knxWWAacJaGqg5OuJh?max-format=0: Proxy Authentication Required

$sudo snap prepare-image ....
error: Get https://api.snapcraft.io/api/v1/snaps/assertions/snap-revision/lUQCdOYc7CNWpZGJ6LrsKNj8CLE7CwE0PUHtCfXxYxtQo3knxWWAacJaGqg5OuJh?max-format=0: net/http: request canceled while waiting for connection (Client.Timeout exceeded while awaiting headers)

I also set proxy in .bashrc, /env/environment, edit snapd.service and still get same results...
Any idea?