Build a static binary

Using

RUSTFLAGS='-C target-feature=+crt-static' cargo build --release

returns this head scratching error:

error: cannot produce proc-macro for `async-trait v0.1.74` as the target `x86_64-unknown-linux-gnu` does not support these crate types

Fix it by specifying the target platform.

RUSTFLAGS='-C target-feature=+crt-static' cargo build --release --target x86_64-unknown-linux-gnu

If the output is still a dynamic binary, it's probably because some dependency *-sys requires dynamic linking such as hyper (links with cURL) or rustls (links with openssl) according to this.