In your config file you have:
But in your URL you are using:
You will need your DNS server to map testapp to the IP address of your server. Or, edit your /etc/hosts file, adding a line such as:
Replace 127.0.0.1 with your server's IP address if you are trying to reach it from a different ...
Docker containers and vHosts aren't the same thing.
You'll use Docker to store WordPress app in a container, this container sharing the system's kernel. You can even store a database in the same container.
You'll use vHosts in Apache/nginx to make your domain pointing to a specific container by passing the entering request on the Web server in the container.
Add this to the end of your 000-default.conf file:
Allow from all
After you change the file, reload Apache and you will be able to access the site.