Dockerfile: v1.29.4 + clone from the git repository

[macbre]

The Mercurial one at https://hg.nginx.org/nginx seems to be out of date.

Changes with nginx 1.29.4                                        09 Dec 2025

    *) Feature: the ngx_http_proxy_module supports HTTP/2.

    *) Feature: Encrypted ClientHello TLS extension support when using
       OpenSSL ECH feature branch; the "ssl_ech_file" directive.
       Thanks to Stephen Farrell.

    *) Change: validation of host and port in the request line, "Host"
       header field, and ":authority" pseudo-header field has been changed
       to follow RFC 3986.

    *) Change: now a single LF used as a line terminator in a chunked
       request or response body is considered an error.

    *) Bugfix: when using HTTP/3 with OpenSSL 3.5.1 or newer a segmentation
       fault might occur in a worker process; the bug had appeared in
       1.29.1.
       Thanks to Jan Svojanovsky.

    *) Bugfix: a segmentation fault might occur in a worker process if the
       "try_files" directive and "proxy_pass" with a URI were used.