Add builder image for reproducible builds and plugin compatibility

[Copilot]

Building GripMock binaries and plugins outside a controlled environment causes runtime incompatibilities: plugin version mismatches, Go toolchain conflicts, libc incompatibilities (glibc vs musl), and CGO symbol resolution errors.

Changes

New builder image (Dockerfile.builder)

• Pins Go 1.25 + Alpine 3.23 matching runtime and go.mod
• Includes full CGO toolchain: gcc, g++, musl-dev, binutils, git
• Sets CGO_ENABLED=1, CC=gcc, CXX=g++

CI/CD updates (.github/workflows/builder.yaml)

• Adds builder-image job before docker job
• Publishes to Docker Hub and GHCR with semver tags
• Uses separate cache scope
• Sets explicit permissions: contents: read, packages: write

Build targets (Makefile)

build-builder:
	docker buildx build --load -f Dockerfile.builder -t ${BUILDER_IMAGE} .

plugins-docker:
	docker run --rm -v $(pwd):/workspace -w /workspace ${BUILDER_IMAGE} sh -c '\
		mkdir -p plugins; \
		for dir in examples/plugins/*; do \
			[ -d $$dir ] && go build -buildmode=plugin -o plugins/$$(basename $$dir).so $$dir/*.go; \
		done'

Documentation (README.md)

• Documents common build failures prevented by builder image
• Provides plugin build examples with Docker
• Shows multi-stage build pattern for custom plugins

Usage

FROM bavix/gripmock-builder:latest AS builder
WORKDIR /app
COPY . .
RUN go build -buildmode=plugin -o custom.so ./plugin/*.go

FROM bavix/gripmock:latest
COPY --from=builder /app/custom.so /plugins/
CMD ["gripmock", "--plugins", "/plugins", "/proto/service.proto"]

Main Dockerfile unchanged to maintain existing build behavior.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create a builder image</issue_title>
<issue_description>Currently, building binaries and plugins outside of a controlled environment may lead to runtime incompatibilities and non-reproducible builds. Introducing a dedicated builder image would provide a stable and consistent toolchain for compilation and linking.

A builder image would ensure:

• Reproducible builds with pinned compiler, Go version, and system libraries
• Correct linking of binaries and plugins
• Fewer environment-specific issues in local builds and CI/CD
• Cleaner production images via multi-stage Docker builds

Common issues without a builder image:

• Plugin / runtime mismatch

  plugin was built with a different version of package runtime
  

• Go toolchain mismatch

  panic: plugin.Open: plugin was built with a different version of package sync
  

• libc incompatibility (glibc vs musl)

  /lib64/libc.so.6: version `GLIBC_2.34' not found
  

• Invalid or incompatible binary format

  invalid ELF header
  

• Missing shared libraries at runtime

  error while loading shared libraries: libstdc++.so.6: cannot open shared object file
  

• CGO symbol resolution errors

  undefined symbol: sqlite3_open_v2
  

Proposed solution:
Provide an official builder image that:

• Pins Go and system dependency versions
• Is used for compiling binaries and plugins
• Is compatible with the recommended runtime image
• Can be easily integrated into CI/CD and multi-stage Dockerfiles

This would significantly improve build reliability and developer experience.
</issue_description>

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

• Fixes Create a builder image #712

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