Support Incremental Builds by not re-publish a container if nothing's changed

[afscrome]

If you run dotnet publish /t:PublishContainer on a project that hasn't changed since the last container publish, the SDK will build a new image, even though nothing's changed

Repro Steps

1. Run dotnet publish /t:PublishContainer
2. Run dotnet publish /t:PublishContainer a second time, without making any other code changes

Expected Results

The second (and subsequent) publish command shouldn't build a whole new container until something else has changed

Actual Results

The second (and subsequent) commands build a new image - note how in the below example the image id has changed between the two publishes.

$> dotnet publish app /t:PublishContainer

  Determining projects to restore...
  All projects are up-to-date for restore.
  Sample -> C:\src\ACTest\artifacts\bin\Sample\Sample.dll
  Sample -> C:\src\ACTest\artifacts\publish\Sample\
  Building image 'actest/myapp' with tags 'latest' on top of base image 'mcr.microsoft.com/dotnet/aspnet:8.0'.
  Pushed image 'actest/myapp:latest' to local registry via 'docker'.

$> docker images actest/myapp
REPOSITORY     TAG       IMAGE ID       CREATED         SIZE
actest/myapp   latest    7bef4d40e5d3   4 seconds ago   221MB

$> dotnet publish app /t:PublishContainer

  Determining projects to restore...
  All projects are up-to-date for restore.
  Sample -> C:\src\ACTest\artifacts\bin\Sample\Sample.dll
  Sample -> C:\src\ACTest\artifacts\publish\Sample\
  Building image 'actest/myapp' with tags 'latest' on top of base image 'mcr.microsoft.com/dotnet/aspnet:8.0'.
  Pushed image 'actest/myapp:latest' to local registry via 'docker'.

$> docker images actest/myapp
REPOSITORY     TAG       IMAGE ID       CREATED         SIZE
actest/myapp   latest    7c293a165f4d   5 seconds ago   221MB