Drop use of buildpack-deps as base image

[mthalman]

• Updates 5.0 SDK buster and focal images to be based on aspnet base image instead of buildpack-deps.
• Adds ca-certificates, curl, git, and wget to 5.0 SDK images for alpine, buster, and focal.
• Adds test to validate the contents of the SDK.
• Adds tests to validate the derivation of the .NET version environment variables.

Fixes #1814
Fixes #1843

[MichaelSimons]

Can you provide a summary of the size impact to this change? What I am looking for is the base image size, final image size and the original image size. Having this for the debian and ubuntu amd64 images would be sufficient.

[mthalman]

Image Size Summary (on disk)

Focal

aspnet Base	Original (based on buildpack-deps)	Final
233 MB	738 MB	630 MB

Buster

aspnet Base	Original (based on buildpack-deps)	Final
209 MB	734 MB	598 MB

[mthalman]

I just realized that buster SDK is now based on buster-slim. I suppose we should change its tag to be buster-slim now?

[MichaelSimons]

I just realized that buster SDK is now based on buster-slim. I suppose we should change its tag to be buster-slim now?

Yes, I think so.

[mthalman]

Here's a comparison of the benefits these changes provide in terms of the download cost savings. The canonical scenario here is a multi-stage Dockerfile that builds and publishes an ASP.NET app (see the sample ASP.NET Dockerfile). The scenario involves first pulling the SDK image in order to build the application and then pulling the ASP.NET image to contain the published application. Before these changes, there was no layer sharing between the ASP.NET image and the SDK image. This was wasteful because the SDK image does contain ASP.NET bits; it just wasn't based on the ASP.NET image. With the changes that were made here, the SDK is now based on the ASP.NET image which means that once you've pulled the SDK image, you now essentially have the ASP.NET image as well. This provides substantial download savings when both images need to be pulled.

Focal

	Before	After
1. Pull SDK image	268 MB	232 MB
2. Pull ASP.NET image	64 MB	10 KB (manifest only)

Net download savings: 100 MB (-30%)

Buster

	Before	After
1. Pull SDK image	280 MB	218 MB
2. Pull ASP.NET image	84 MB	4 KB (manifest only)

Net download savings: 146 MB (-40%)