The `docker/go-connections` package was only used for a quite generic utility.
This patch removes the use of the package by replacing the `GetProxyEnv` utility with
a local function that's based on the one in golang.org/x/net/http/httpproxy:
c21de06aaf/http/httpproxy/proxy.go (L100-L107)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>