fix docker build · aramperes/onetun@f3661c0

unused import: `std::collections::HashSet`

unused import: `std::fs::read_to_string`

constant `DEFAULT_PORT_FORWARD_SOURCE` is never used

function `parse_addr` is never used

function `parse_ip` is never used

function `parse_private_key` is never used

function `parse_public_key` is never used

function `parse_preshared_key` is never used

function `parse_keep_alive` is never used

function `parse_mtu` is never used

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