AH01144: No protocol handler was valid for the URL /

I spend an hour over this confusing error message and I hope this will save you some time!

<VirtualHost *:80>
	ServerName foo.example.com
	ProxyPass / https://localhost:81/
	ProxyPassReverse / https://localhost:81/
</VirtualHost>

What’s the issue?! SSL! Your Docker container is probably not listening off SSL. The fix:

<VirtualHost *:80>
	ServerName foo.example.com
	ProxyPass / http://localhost:81/
	ProxyPassReverse / http://localhost:81/
</VirtualHost>