Sure, you could add another container. Personally, however, I would just install nginx directly on the droplet, have it redirect and/or proxy requests for *.mydomain.tld on port 80 to the wordpress container listening on some other upstream_port on the loopback interface.

Step 1 - Install Nginx.

apt-get update && apt-get install nginx

Step 2 - Disable the default site.

rm /etc/nginx/sites-enabled/*
rm /etc/nginx/conf.d/*

Step 3 - Create a new virutual host.

nano /etc/nginx/sites-available/mydomain_proxy

server {
    listen 80 default_server;
    return 444;
}

server {
    listen 80;
    server_name *.mydomain.tld mydomain.tld;
    return 301 http://blog.mydomain.tld$request_uri;
}

server {
    listen 80;
    server_name blog.mydomain.tld;
    location / {
        proxy_redirect     off;
        proxy_set_header   Host $host;
        proxy_set_header   X-Real-IP $remote_addr;
        proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header   X-Forwarded-Host $server_name;
        proxy_pass         http://127.0.0.1:upstream_port;
    }
}

Step 4 - Enable the new site.

ln -s /etc/nginx/sites-available/mydomain_proxy /etc/nginx/sites-enabled

Step 5 - Restart nginx

service nginx restart

Step 6 - Modify DNS records.

Add a CNAME record for host * that points to any host.mydomain.tld. for which there is an A record corresponding to the nginx server’s public IP address.

Step 7 - Open port 80

If you haven’t done so already, allow trafic to port 80 from the world. Use UFW for simplicity.

ufw enable && ufw allow 80/tcp

Now any request for hosts other than blog.mydomain.tld will redirect to blog.mydomain.tld. If you must use a container, do the exact same thing, and be sure to link the appropriate ports.