http_proxy support broken with placeholders

Issue Details

When using a http proxy like squid, with the change introduced in https://github.com/caddyserver/caddy/pull/6955 in 2.10.1, TLS handshakes with the http proxy fail like:

{"level":"error","ts":1756479775.99738,"logger":"http.log.access","msg":"handled request","request":{"remote_ip":"10.32.0.1","remote_port":"5970","client_ip":"10.32.0.1","proto":"HTTP/1.1","method":"HEAD","host":"REDACTED","uri":"REDACTED","headers":{"Authorization":["REDACTED"],"Julia-Version":["1.11.5"],"Accept":["*/*"],"User-Agent":["HTTP.jl/1.11.5"],"Content-Length":["0"],"Accept-Encoding":["gzip"]}},"bytes_read":0,"user_id":"","duration":0.017466018,"size":0,"status":503,"resp_headers":{"Mime-Version":["1.0"],"Content-Length":["4064"],"Content-Language":["en"],"X-Cache-Lookup":["NONE from ip-172-31-1-132.ec2.internal:3128"],"Date":["Fri, 29 Aug 2025 15:02:55 GMT"],"Via":["1.1 Caddy","1.1 ip-172-31-1-132.ec2.internal (squid/4.15)"],"Content-Type":["text/html;charset=utf-8"],"X-Squid-Error":["ERR_SECURE_CONNECT_FAIL 71"],"X-Cache":["MISS from ip-172-31-1-132.ec2.internal"],"Vary":["Accept-Language"]}}

Notice the ERR_SECURE_CONNECT_FAIL. When changing the Caddyfile to not use placeholders, it works fine.

Assistance Disclosure

AI used

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Claude helped find the change between 2.10.0 and 2.10.2 that broke it.