[Samples] Update IIS sample Dockerfile

[lucaspimentel]

Summary of changes

Improve the Dockerfile used to install dd-trace-dotnet in the containerized IIS app sample in tracer/samples/IISInDocker.

Reason for change

A user reported having issues with the Dockerfile, and I was able to reproduce.

Implementation details

• move install script to a separate ps1 file
• use absolute paths for msi download and log file (this is the main reason for this PR, the relative paths fail in common scenarios)
• use PowerShell's built-in Invoke-WebRequest instead of .NET's System.Net.WebClient.DownloadFile()
  (which is obsolete)
• get the github release flagged as latest instead of the most recent (not the same thing! this would've downloaded preview versions of v3.x, for example)
• add more verbosity in console messages (show url, download path, log path, etc) to ease troubleshooting
• add basic .dockerignore file

Test coverage

Tested manually.

[andrewlock]

I take your word for it, but I don't really understand why we need to make some of these changes 🤷‍♂️

[andrewlock]

Is there a reason this can't be done here and instead has to be moved to a file? 🤔 I don't know of a good reason it needs to be?

[lucaspimentel]

Level of care low on this one, but I can think of a few very "mild" reasons 😅

• If a script is going to be more than a handful of lines, I think it deserves its own file (cleaner code, separation of concerns, etc)
• This particular script is not Docker-specific. A separate file makes it easier to copy and use elsewhere.
• It definitely made it easier and faster for me to fix it outside of Docker. (probably the main reason I did it)