external-proxy.conf

server {
    listen 80;
    server_name mysite.com;

    location /.foo {
        alias /opt/foo;
        try_files $uri @proxy;
    }

    location @proxy {
        proxy_set_header X-Real-IP  $remote_addr;
        proxy_set_header X-Forwarded-For $remote_addr;
        proxy_set_header Host $host;

        proxy_pass http://internal-proxy;

        proxy_http_version 1.1;
        proxy_buffering off;
        proxy_buffer_size 4k;
    }
}

internal-proxy.conf

server {
    listen 80;
    server_name mysite.com;

    location /.foo {
        root /opt;
    }

    location / {
        root /opt/root;
    }
}

nginx.conf

user                            nginx;
worker_processes                1;

error_log                       /var/log/nginx/error.log warn;
pid                             /var/run/nginx.pid;

events {
    worker_connections          1024;
}

http {
    include                     /etc/nginx/mime.types;
    default_type                application/octet-stream;
    sendfile                    on;
    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';
    access_log                  /var/log/nginx/access.log main;
    keepalive_timeout           3000;
    client_max_body_size        0;

    include /etc/nginx/conf.d/*.conf;
}

Proxies executed as containers:

docker run \
  --name internal-proxy -d \
  -v $(pwd)/nginx.conf:/etc/nginx/nginx.conf:ro \
  -v $(pwd)/int.conf:/etc/nginx/conf.d/int.conf:ro \
nginx:1.13-alpine && \
docker run --name ext-proxy \
  --link internal-proxy:internal-proxy -d \
  -v $(pwd)/nginx.conf:/etc/nginx/nginx.conf:ro \
  -v $(pwd)/ext.conf:/etc/nginx/conf.d/ext.conf:ro \
  -p 8888:80 \
nginx:1.13-alpine

I create files on both proxies:

docker exec -ti int-proxy ash -c 'mkdir -p /opt/.foo && echo FOO > /opt/.foo/index.html && chown -R nginx:nginx /opt'

docker exec -ti ext-proxy ash -c 'mkdir -p /opt/foo && echo FOOBAR > /opt/foo/index.html && chown -R nginx:nginx /opt'

http://mysite.com:8888/.foo/ returns: FOO and http://mysite.com:8888/.foo/index.html returns FOOBAR.