Set proxy_ssl_trusted_certificate.

Use the CA certificate bundle. For proxying secure services (like using
Vouch).

conf.d/global.conf

 # The resolver for the Docker network.
-resolver        127.0.0.11 valid=30s;
-gzip            on;
-tcp_nopush      on;
-tcp_nodelay     on;
-server_tokens   off;
-include         snippets/common-headers.conf;
+resolver                        127.0.0.11 valid=30s;
+gzip                            on;
+tcp_nopush                      on;
+tcp_nodelay                     on;
+server_tokens                   off;
+proxy_ssl_trusted_certificate   /etc/ssl/certs/ca-certificates.crt;
+include                         snippets/common-headers.conf;