feat: Add Docker build context

[HofmeisterAn]

What does this PR do?

This PR adds WithContextDirectory(string) to ImageFromDockerfileBuilder, letting developers specify the Docker build context directory. Previously, Testcontainers for .NET required the Dockerfile and .dockerignore to be inside the build context, which isn't always necessary/practical. The Docker CLI allows specifying a separate build context for a Dockerfile.

For backward compatibility, developers still need to specify the Dockerfile directory and name. We may refactor this in a follow-up PR. If no build context is specified, Testcontainers for .NET defaults to using the Dockerfile directory as the build context.

Additionally, this PR adds support for Dockerfile-specific ignore files. If such a file exists in the Dockerfile directory, it will be used instead of .dockerignore.

Why is it important?

-

Related issues

• Closes [Enhancement]: Allow the Docker Image Build Context Path to Be Different From the Dockerfile Path #1514
• Closes [Enhancement]: Support for custom .dockerignore files #1182

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