fix: Trim relative path indicators from Dockerfile path

[HofmeisterAn]

What does this PR do?

The pull request removes the relative path indicators from the Dockerfile path in the WithDockerfile(string) image builder API.

Why is it important?

This change is necessary to prevent issues when a developer passes a relative path to the API, such as ./Dockerfile. If the Dockerfile path starts with a relative path indicator, the .dockerignore negate pattern that makes sure the Dockerfile is always included in the tarball might not work correct. This can result in the Dockerfile not being included. Building the image fails with the following or a similar error:

Cannot locate specified Dockerfile: ./Dockerfile

Trimming the relative path indicators prevents a follow-up issue. If the path contains relative path indicators, setting the ImageBuildParameters.Dockerfile property causes the build to fail as well.

Related issues

• Closes [Bug]: Unable to locate Dockerfile on v3.8.0 #1143

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