feat: add initial Earthfile and README for Buildkit container image

containers/buildkit/Earthfile

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+VERSION 0.8
+IMPORT .. AS common
+
+ARG --global BASE_VERSION
+ARG --global TAG
+ARG --global GITHUB_REPOSITORY_OWNER
+
+buildkit-base:
+    FROM docker.io/moby/buildkit:${BASE_VERSION}
+    DO common+ADD_LABELS
+
+validate:
+    FROM +buildkit-base
+    RUN --no-cache test -n "$BASE_VERSION" || (echo "BASE_VERSION is required" && exit 1)
+
+build:
+    BUILD +validate
+    FROM +buildkit-base
+    DO common+SAVE_IMAGE --IMAGE_NAME=buildkit --TAG=${TAG} --GITHUB_REPOSITORY_OWNER=${GITHUB_REPOSITORY_OWNER}
+
+build-multiplatform:
+    DO --pass-args common+BUILD_MULTIPLATFORM --TARGET=./buildkit+build

containers/buildkit/README.md

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+# buildkit
+
+Aliased container image for Buildkit.
+
+## Available Versions
+
+<!-- VERSIONS_START -->
+
+| Tag               | Base Version       |
+| ----------------- | ------------------ |
+| `buildx-stable-1` | buildx-stable-1    |
+
+<!-- VERSIONS_END -->
+
+## Usage
+
+```bash
+docker pull ghcr.io/expnt/containers/buildkit:<tag>
+```

containers/buildkit/versions.yaml

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+image_name: buildkit
+tag_pattern: "{base_version}"
+versions:
+  - base_version: "buildx-stable-1"
+watch:
+  - pattern: ^buildx-stable-(\d+)$
+    source: docker.io/moby/buildkit
+    target: base_version
+    type: docker