[ECS] Add setting for ReservedPorts/ReservedPortsUDP

[samuelkarp]

Originally requested by @chathsuom in #815 (comment).

The ReservedPorts and ReservedPortsUDP fields cause the ECS agent to restrict tasks that use those ports from being placed on a given container instance.

[chathsuom]

Thanks @samuelkarp
Use case behind this is, we use a custom SSH container which has some additional functionality (MFA etc.) which I cannot do with the admin controller. (Not so sure I am correct here though)
Next step, I will try if I can customise the admin controller to include our requirements.

[samuelkarp]

If you want to replace the admin container and use yours instead, you can override the image used for the admin container through the settings.host-containers.admin.source setting. If you want to customize and extend the default admin container image, you can find the source for it here.

[samuelkarp]

You can use any image you want, but our default one has some useful things you might want to include in your image. You mentioned that you've got your own SSH config, but for reference we use this script as the default command to start the container (which grabs the right public key and runs sshd) and this SSH config. We also include a statically-linked bash and a script called sheltie that effectively gives you a shell "on the host".

[stmcginnis]

Anyone know if this is still needed? This issue has been open for some time now, and I don't think I've seen other requests for this. Just trying to clean up our backlog of issues, and if this isn't something that has been a big enough concern to address in the last 2+ years I'm wondering if it makes sense to just close it.