Throw the new DockerUnavailableException when Docker is not available

[0xced]

What does this PR do?

This pull request introduces a new DockerUnavailableException which is thrown instead of ArgumentException which was not the appropriate exception to throw since the root of the problem is not a bad argument but the state of the system which is wrong.

The exception message has also been slightly improved.

Why is it important?

Unlike ArgumentException, DockerUnavailableException can be caught handled.

Related issues

Once both this pull request and #1263 are merged, the error messages will be improved, giving a better hint to the user at what went wrong. Here are two sample messages from the new DockerUnavailableException.

Running Testcontainers after quitting Docker Desktop:

Docker is either not running or misconfigured. Please ensure that Docker is available at unix:///var/run/docker.sock or unix:///Users/0xced/.docker/run/docker.sock
You can customize your configuration using either the environment variables or the ~/.testcontainers.properties file. For more information, visit:
https://dotnet.testcontainers.org/custom_configuration/

Running Testcontainers after quitting OrbStack:

Docker is either not running or misconfigured. Please ensure that Docker is available at unix:///var/run/docker.sock or unix:///Users/0xced/.orbstack/run/docker.sock
You can customize your configuration using either the environment variables or the ~/.testcontainers.properties file. For more information, visit:
https://dotnet.testcontainers.org/custom_configuration/

How to test this PR

Quit Docker Desktop or configure a bad endpoint then try to run a container. Observe the improved exception and error message.

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[HofmeisterAn]

Maybe we should discuss this in an issue. We can merge the PR, but I think these features would be better in Docker.DotNet. Things like detecting the container runtime, getting private registry credentials, etc. should be part of Docker.DotNet where more developers benefit from. This is one reason why we forked Docker.DotNet and use our own implementation. The Java's client has way more features and is much more developer-friendly than our .NET counterpart.