Cannot download Docker images behind a proxy

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Here is a link to the official Docker documentation for proxy HTTP:
https://docs.docker.com/config/daemon/systemd/#httphttps-proxy

A quick outline:

First, create a systemd drop-in directory for the Docker service:

mkdir /etc/systemd/system/docker.service.d

Now create a file called /etc/systemd/system/docker.service.d/http-proxy.conf that adds the HTTP_PROXY and HTTPS_PROXY environment variables:

[Service]
Environment="HTTP_PROXY=http://proxy.example.com:80/"
Environment="HTTPS_PROXY=http://proxy.example.com:80/"

If you have internal Docker registries that you need to contact without proxying you can specify them via the NO_PROXY environment variable:

Environment="HTTP_PROXY=http://proxy.example.com:80/"
Environment="HTTPS_PROXY=http://proxy.example.com:80/"
Environment="NO_PROXY=localhost,127.0.0.0/8,docker-registry.somecorporation.com"

Flush changes:

$ sudo systemctl daemon-reload

Verify that the configuration has been loaded:

$ sudo systemctl show --property Environment docker
Environment=HTTP_PROXY=http://proxy.example.com:80/
Environment=HTTPS_PROXY=http://proxy.example.com:80/

Restart Docker:

$ sudo systemctl restart docker

Footnote regarding HTTP_PROXY vs. HTTPS_PROXY: for a long time, setting HTTP_PROXY alone has been good enough. But with version 20.10.8, Docker has moved on to Go 1.16, which changes the semantics of this variable:
https://golang.org/doc/go1.16#net/http

For https:// URLs, the proxy is now determined by the HTTPS_PROXY variable, with no fallback on HTTP_PROXY.

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