Add Evol testbed CI image

ci/README.md

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+# Gitea Actions CI image
+
+This image extends Gitea/act's Ubuntu runner image:
+
+```text
+catthehacker/ubuntu:act-latest
+```
+
+It includes:
+
+- Clang/LLVM 22
+- Meson
+- Ninja
+- Python 3 + pip + venv
+- Vulkan development packages/tools
+- Git/build-essential/pkg-config
+- Node/runtime pieces inherited from the act Ubuntu image, so JavaScript actions like `actions/checkout` keep working
+
+## Files prepared
+
+```text
+ci/Dockerfile
+.dockerignore
+.gitea/workflows/build-ci-image.yml
+.gitea/workflows/build.yml
+```
+
+## Required Gitea variables/secrets
+
+In your repository or organization settings, add these **Actions variables**:
+
+```text
+REGISTRY_HOST=git.neosisyphus.com
+REGISTRY_IMAGE=git.neosisyphus.com/evol3d/evol-testbed
+CI_IMAGE=git.neosisyphus.com/evol3d/evol-testbed
+```
+
+Add these **Actions secrets**:
+
+```text
+REGISTRY_USERNAME=your-gitea-username
+REGISTRY_PASSWORD=your-gitea-token-or-password
+```
+
+The token/password needs permission to publish packages/container images.
+
+## Build manually
+
+```bash
+docker build -t evol-testbed:latest -f ci/Dockerfile .
+```
+
+Test:
+
+```bash
+docker run --rm -it evol-testbed:latest bash -lc '
+  clang --version &&
+  meson --version &&
+  ninja --version &&
+  python3 --version &&
+  vulkaninfo --summary || true
+'
+```
+
+Tag and push:
+
+```bash
+docker login git.neosisyphus.com
+
+docker tag evol-testbed:latest git.neosisyphus.com/evol3d/evol-testbed:latest
+docker tag evol-testbed:latest git.neosisyphus.com/evol3d/evol-testbed:clang22
+
+docker push git.neosisyphus.com/evol3d/evol-testbed:latest
+docker push git.neosisyphus.com/evol3d/evol-testbed:clang22
+```
+
+## Build automatically in Gitea Actions
+
+After setting the variables/secrets above, run:
+
+```text
+.gitea/workflows/build-ci-image.yml
+```
+
+It builds and pushes:
+
+```text
+${REGISTRY_IMAGE}:latest
+${REGISTRY_IMAGE}:clang22
+```
+
+## Use in normal builds
+
+`.gitea/workflows/build.yml` is already prepared to use:
+
+```yaml
+container:
+  image: ${{ vars.CI_IMAGE }}:clang22
+```
+
+If expressions do not work in `container.image` on your Gitea version, replace it with the literal image path:
+
+```yaml
+container:
+  image: git.neosisyphus.com/evol3d/evol-testbed:clang22
+```