fix: Do not pre pull Dockerfile build stages that do not correspond to base images

[HofmeisterAn]

What does this PR do?

This pull request excludes Dockerfile build stages that do not correspond to base images.

Why is it important?

This pull request addresses an issue related to building Docker images that have dependencies on base images from private registries. Currently, Testcontainers encounters an issue when attempting to pull actual images. Testcontainers does not distinguish between base images and intermediate build stages. Intermediate build stages cannot be pulled; Testcontainers must ignore these stages. E.g. the second line of the following Dockerfile:

FROM mcr.microsoft.com/dotnet/sdk:6.0 AS build
FROM build

Related issues

• Closes [Bug]: ImageFromDockerfileBuilder cannot build Dockerfile with intermediate layer (regression works with 3.3.0) #972
• Relates [Enhancement]: Resolve variables (ARGs) during Docker image build #980

Follow-ups

Testcontainers for .NET cannot resolve variables (ARGs) during the Docker image build process. Currently, Testcontainers ignores all base images that utilize variables. To pull these base images in advance too, Testcontainers needs to have the capability to resolve the default values or build arguments of variables.

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