Add sudo -E flag to curl command for proxy support

[Wenzel]

Description

The curl command needs the -E flag with sudo to preserve environment variables, particularly HTTP_PROXY and HTTPS_PROXY settings. Without this flag, users behind corporate proxies cannot download Docker's GPG key.

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[thaJeztah]

Wondering if we should change this so that curl isn't executed as root;

curl -fsSL "{{% param "download-url-base" %}}/gpg" | sudo tee /etc/apt/keyrings/docker.asc > /dev/null

or with an intermediate file, but that requires a cleanup after;

curl -fsSL "{{% param "download-url-base" %}}/gpg" -o docker.asc
sudo install -o root -g root -m 0644 docker.asc /etc/apt/keyrings/docker.asc
rm -f docker.asc

@dvdksn @vvoland any thoughts? If we change, we should also update the debian.md accordingly

[dvdksn]

Hmm, we previously agreed that sudo curl was an acceptable tradeoff - see https://github.com/docker/docs/pull/19138/files#r1467055019

I think sudo -E curl still seems OK to me but no strong opinions. If not sudo curl then I'd go for sudo tee > /dev/null

[vvoland]

Not a big deal I think, but if we want something shorter than tee we could use: sudo cat - >/etc/apt/keyrings/docker.asc

or sudo cat ->/etc/apt/keyrings/docker.asc if we want to be funny and use the cat goes to operator (which is a close relative to the "downto" operator in C)