Adding dotnet:4-alpine image

[BaileyFirman]

Adding alpine support for dotnet, motivated by the large number of image vulnerabilities mostly reported for Debian.
This is a POC PR to hopefully get the ball rolling on this issue: #901 with changes I based off #924

Note: I am using the contents of this new Dockerfile internally instead of FROM mcr.microsoft.com/azure-functions/dotnet:4 in order to meet security requirements, at time of writing there are no vulnerabilities reported when scanning the resulting image.

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• New Unit tests were added for the changes made and CI is passing.

[BaileyFirman]

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[SymbioticKilla]

Any ETA?

[SymbioticKilla]

@Tonewall @ahmelsayed @vrdmr @TsuyoshiUshio @pragnagopa @anirudhgarg @balag0 @CooperLink @amamounelsayed @VpOfEngineering

Hi guys!

Cann it be more prioritized?
Debian images have up-to 300 CVEs. It is very hard to check each of them in term of security.
Thanks!