Install this with ddev get drud/ddev-proxy-support
It installs a docker-compose.proxy.yaml and pre.Dockerfile.proxy-support which add proxy capabilities to DDEV v1.19.5+
With this setup you should be able to use webimage_extra_packages against a proxy, and you should also be able to ddev ssh into the web container and use curl and see it using the proxy. curl -v -I <target> is a good test. See also the tests.
Details about how to lab-test this are in Lab-testing a proxied environment.
You can also find public proxies of varying reliability at spys and other places, and of course you would never trust them with any traffic, but they're useful for testing.
Contributed and maintained by @rfay based on the original ddev-contrib recipe.