Fail to build image for Dockerfile with the default image "7.0-focal"

[v-xinda]

OS: All
Build Version: 20221014.1
.NET Version: 7.0.100-rc.2.22477.23

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]

Same as #3512. @dbreshears, do you want to include this in the 1.22.2 servicing update? It's a straightforward, low-risk fix. The workaround the user can do is to take -focal off when using .NET 7 previews.

[dbreshears]

@bwateratmsft , yes let's fix this in 1.22.2

[bwateratmsft]

For posterity, the fix is to stop doing -focal with the image name scaffolding. From here on it will just be :6.0, :7.0, etc.

[bwateratmsft]

A fix for this issue has been released in Docker extension version 1.22.2.