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Docker uses the HTTP_PROXY environment variable if present, for example:
HTTP_PROXY=http://192.168.1.1:3128/ docker pull busybox
But instead, I suggest you have a look at your /etc/default/docker configuration file : you should have a line to uncomment (and maybe adjust) to get your proxy settings applied automatically.
# If you need Docker to use an HTTP proxy, it can also be specified here. #export http_proxy="http://127.0.0.1:3128/"
Then restart the Docker server:
service docker restart