As an opportunity to rewire my brain from "docker" to "podman" and "buildah" I started to create an image build with an ECH enabled curl at https://gitlab.com/hoexter-experiments/ech.

Not sure if it helps anyone, but setup should be like this:

git clone https://gitlab.com/hoexter-experiments/ech
cd ech
buildah build --layers -f Dockerfile -t echtest
podman run -ti echtest /usr/local/bin/curl \
  --ech true --doh-url https://one.one.one.one/dns-query \
  https://crypto.cloudflare.com/cdn-cgi/trace.cgi
fl=48f121
h=crypto.cloudflare.com
ip=2.205.251.187
ts=1773410985.168
visit_scheme=https
uag=curl/8.19.0
colo=DUS
sliver=none
http=http/2
loc=DE
tls=TLSv1.3
sni=encrypted
warp=off
gateway=off
rbi=off
kex=X25519

It also builds nginx and you can use that for a local test within the image. More details in the README.