Accessing Postgresql via deadpool_postgres, tokio_postgres and rustls

Question

I try to use deadpool_postgres to access a Postgres database from my Axum web app. I want/need to use a Postgres connection string in URL format to connect, which seems not to be supported by deadpool-postgres. So I have to use the Config object of the underlying tokio_postgres directly.

This worked fine for my local test database, but on the target system (in this case: a DigitalOcean App and hosted Postgres) I use sslmode=require. So I have to setup TLS for my database connection. Here is my code so far:

async fn create_pool(connection_string: &str) -> anyhow::Result<Pool> {
    let pg_config = tokio_postgres::Config::from_str(connection_string)?;
    let mgr_config = ManagerConfig {
        recycling_method: RecyclingMethod::Fast
    };
    // https://docs.rs/rustls/latest/rustls/#getting-started
    let mut root_store = rustls::RootCertStore::empty();
    root_store.add_trust_anchors(
        webpki_roots::TLS_SERVER_ROOTS
            .iter()
            .map(|ta| {
                rustls::OwnedTrustAnchor::from_subject_spki_name_constraints(
                    ta.subject,
                    ta.spki,
                    ta.name_constraints,
                )
            })
    );
    // https://crates.io/crates/tokio-postgres-rustls
    let config = rustls::ClientConfig::builder()
        .with_safe_defaults()
        .with_root_certificates(root_store)
        .with_no_client_auth();
    let tls = tokio_postgres_rustls::MakeRustlsConnect::new(config);
    let mgr = Manager::from_config(pg_config, tls, mgr_config);
    Ok(Pool::builder(mgr).max_size(16).build()?)
}

In my logs I see:

Client auth requested but no cert/sigscheme available    
Error occurred while creating a new object: error performing TLS handshake: invalid peer certificate: UnknownIssuer

I have basic knowledge about SSL/TLS, but not too deep. I can connect to the hosted database via psql from my local machine, so no special certificate should be required.

Any hint how to get this working would be highly appreciated!