feat: Add FLAGD_BASE_IMAGE build argument to Dockerfile

[Copilot]

Enables building the testbed image with custom flagd base images for pipeline testing scenarios.

Changes

• Dockerfile: Added FLAGD_BASE_IMAGE build argument, defaults to ghcr.io/open-feature/flagd:v0.12.9
• README: Documented build argument usage with examples

Usage

# Default behavior (unchanged)
docker build -f flagd/Dockerfile -t flagd-testbed:latest --target testbed .

# With custom flagd image
docker build -f flagd/Dockerfile \
  --build-arg FLAGD_BASE_IMAGE=ghcr.io/open-feature/flagd:v0.13.0 \
  -t flagd-testbed:custom --target testbed .

Existing builds and CI workflows continue to work without modification.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Allow passing a different base image for flagd in Dockerfile</issue_title>
<issue_description>## Background
Currently, the Dockerfile in the flagd-testbed repository uses a fixed base image for building the testbed image. In order to facilitate testing with custom or pipeline-built flagd images, we need the ability to specify a different base image when building the testbed container.

Proposal

• Modify the Dockerfile to accept an argument (e.g., FLAGD_BASE_IMAGE) allowing the user to specify a custom base image.
• Update any relevant documentation to explain how to use this new argument during the build process.

Motivation

This change will enable building the testbed image in the flagd pipelines using a base image created within the same pipeline, supporting more flexible and robust testing scenarios.

Acceptance Criteria

• The Dockerfile accepts a build argument for the base flagd image
• Documentation is updated to describe how to use the argument
• Backwards compatibility is maintained (defaults to current base image if not provided)

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Feel free to suggest alternative approaches or raise concerns about the impact on current workflows.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Allow passing a different base image for flagd in Dockerfile #307

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