The number of open-source DoH proxies available has exploded since the DoH was standardized. Some notable implementations that should be mentioned here are:
Cloudflare's cloudflared - while this is specifically meant to be used with 126.96.36.199, the daemon can be used with any DoH end-point by specifying proxy-dns-upstream to a different end-point.
DNSCrypt-Proxy by Frank Denis - this is quite flexible implementation of DNS proxy as it supports DNS-over-HTTPS (DoH) using TLS 1.3, DNSCrypt and Anonymized DNS.
doh-proxy by Facebook - the doh-stub part of the project can act as a stub resolver for local DNS clients
doh-proxy in Rust by Frank Denis
doh-proxy by Facebook - the doh-proxy (HTTP/2) and doh-httpproxy (HTTP/1) parts of the project can be put in front of recursive DNS service