Fail to build image for Dockerfile with the default 7.0 image

[v-ruizh]

OS: Win 10
Docker Extension Build Version: 20220506.1
.NET Version: 7.0.100-preview.3.22179.4

Repro Steps:

1. Create and open a .NET 7.0 MVC project in VS Code.
2. Add Docker files to this project (Linux Container) -> Right click the "Dockerfile" file -> Select "Build Image...".
3. Check whether succeeds to build image of the "Dockerfile" file.

Expect:
Succeed to build image of the "Dockerfile" file.

Actual:
Fail to build image of the "Dockerfile" file.

More Info:

1. This issue only reproduces with the default Docker image "7.0-focal".
2. This issue does not reproduce with other images "7.0 and 7.0-alpine".
3. This issue does not reproduce for a .NET 6.0 project.

[bwateratmsft]

We have updated the scaffolding to return to just mcr.microsoft.com/dotnet/aspnet:7.0. It will no longer do the erroneous mcr.microsoft.com/dotnet/aspnet:7.0-focal. This applies to all .NET scaffolding including 6.0 and 3.1.

In the past we found that the Debian images (e.g. :6.0) were lacking in some binaries needed to support the attach scenario, but the Ubuntu images (e.g. :6.0-focal) did have them, so we changed to those. It seems the C# extension has changed some things and those binaries are no longer needed, so we have returned to the Debian images.

As a workaround to any customers hitting this, you can simply remove the -focal from the two images in the Dockerfile.

[bwateratmsft]

Since we'll be fixing #3647 in 1.22.2, I'm going to mark this one as a duplicate of that.