Modify get_unavailable_ports to avoid KeyError: 'PublicPort'

[HatsuMora]

I was running CTFd in a docker host that had portainer.io running at the same time. And I got an error message containing KeyError: 'PublicPort'.

Once I looked into https://my-https-docker:2376/containers/json?all=1 I could see why the error happened.

The portainer ports looks like this:

"Ports":[
{
"IP":"0.0.0.0",
"PrivatePort":9443,
"PublicPort":9443,
"Type":"tcp"
},
{
"IP":"0.0.0.0",
"PrivatePort":8000,
"PublicPort":5080,
"Type":"tcp"
},
{
"PrivatePort":9000,
"Type":"tcp"
}.

Adding an extra check to verify that the Ports contain the key PublicPort