Snap (Ubuntu package management)
Install
sudo apt install -y snapd
CLI
snap find
snap install
Configure proxy
sudo mkdir -p /etc/systemd/system/snapd.service.d/
echo -e '[Service]\nEnvironment="http_proxy=http://proxy.example.com:3128/"' | sudo tee /etc/systemd/system/snapd.service.d/http-proxy.conf
echo -e '[Service]\nEnvironment="https_proxy=http://proxy.example.com:3128/"' | sudo tee /etc/systemd/system/snapd.service.d/https-proxy.conf
sudo systemctl daemon-reload
sudo systemctl restart snapd
# debug proxy
systemctl show snapd | grep proxy