Increase proxy_ssl_verify_depth to handle intermediate certificates

[calvincheng8]

Receiving SSL error now since an intermediate certificate is added to docker.io

2026/06/15 19:12:14 [error] 75#75: *11 upstream SSL certificate verify error: (22:certificate chain too long) while SSL handshaking to upstream, client: 127.0.0.1, server: proxy_caching_, request: "GET /token?scope=repository%3Alibrary%2Fubuntu%3Apull&service=registry.docker.io HTTP/1.1", upstream: "https://104.18.43.178:443/token?scope=repository%3Alibrary%2Fubuntu%3Apull&service=registry.docker.io", host: "auth.docker.io"
2026/06/15 19:12:14 [warn] 75#75: *11 upstream server temporarily disabled while SSL handshaking to upstream, client: 127.0.0.1, server: proxy_caching_, request: "GET /token?scope=repository%3Alibrary%2Fubuntu%3Apull&service=registry.docker.io HTTP/1.1", upstream: "https://104.18.43.178:443/token?scope=repository%3Alibrary%2Fubuntu%3Apull&service=registry.docker.io", host: "auth.docker.io"

Increasing the proxy_ssl_verify_depth to 3, the problem is fixed.

{"access_time":"15/Jun/2026:19:21:50 +0000","upstream_cache_status":"HIT","method":"HEAD","uri":"/v2/library/ubuntu/manifests/24.04","request_type":"manifest-secondary","status":"200","bytes_sent":"0","upstream_response_time":"","host":"registry-1.docker.io","proxy_host":"registry-1.docker.io","upstream":""}
{"access_time":"15/Jun/2026:19:21:50 +0000","upstream_cache_status":"","method":"GET","uri":"/v2/library/ubuntu/referrers/sha256:01a14a568a5c77390e74eefc7a2106206f4605338cb7e86e8bf06a18452b5169","request_type":"unknown","status":"401","bytes_sent":"157","upstream_response_time":"0.328","host":"registry-1.docker.io","proxy_host":"registry-1.docker.io","upstream":"54.234.221.194:443"}
{"access_time":"15/Jun/2026:19:21:50 +0000","upstream_cache_status":"","method":"GET","uri":"/token","request_type":"unknown","status":"200","bytes_sent":"5421","upstream_response_time":"0.131","host":"auth.docker.io","proxy_host":"auth.docker.io","upstream":"104.18.43.178:443"}
{"access_time":"15/Jun/2026:19:21:51 +0000","upstream_cache_status":"","method":"GET","uri":"/v2/library/ubuntu/referrers/sha256:01a14a568a5c77390e74eefc7a2106206f4605338cb7e86e8bf06a18452b5169","request_type":"unknown","status":"200","bytes_sent":"89","upstream_response_time":"0.340","host":"registry-1.docker.io","proxy_host":"registry-1.docker.io","upstream":"3.238.169.28:443"}

Fixes #181

[ChristianCiach]

Pinging @rpardini for attention. This is an important fix.