WIP/POC: Initial commit.

Dockerfile

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+FROM gcr.io/go-containerregistry/crane as crane
+FROM alpine
+COPY --from=crane /ko-app/crane /usr/local/bin/crane
+RUN apk add --no-cache jq

ci/pipeline.yml

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+---
+resources:
+- name: source
+  type: git
+  source:
+    uri: https://gitea.redxef.at/redxef/concourse-oci-resource
+    branch: master
+    fetch_tags: true
+- name: upstream-image
+  type: registry-image
+  source:
+    repository: alpine
+
+jobs:
+- name: build-push
+  plan:
+  - get: source
+    trigger: true
+  - get: upstream-image
+    trigger: true
+  - task: compute-docker-tags
+    config:
+      platform: linux
+      image_resource:
+        type: registry-image
+        source:
+          repository: alpine/git
+      inputs:
+      - name: source
+        path: .
+      outputs:
+      - name: docker-tags
+      run:
+        path: sh
+        args:
+        - -c
+        - |
+          #!/usr/bin/env sh
+          git rev-parse --short HEAD > docker-tags/tags.txt
+          git show-ref --tags | \
+              sed -n "/$(git rev-parse HEAD)/ s|$(git rev-parse HEAD).refs/tags/||gp" \
+              >> docker-tags/tags.txt
+  - task: build
+    privileged: true
+    config:
+      platform: linux
+      image_resource:
+        type: registry-image
+        source:
+          repository: redxef/concourse-buildkit
+          tag: v0.2.1
+      inputs:
+      - name: source
+        path: .
+      - name: docker-tags
+      params:
+        username: ((docker.username))
+        password: ((docker.password))
+        repository: docker.io/redxef/concourse-oci-resource
+        tag: latest
+        additional_tags: docker-tags/tags.txt
+        push: true
+        platform: aarch64,arm,ppc64le,s390x,x86_64
+        context: .
+        manual: false
+      run:
+        path: build

src/check

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+#!/usr/bin/env sh
+
+set -eu
+
+. ./common
+
+jq -r tostring << EOF
+[
+    $(crane digest "$REPOSITORY:$TAG")
+]
+EOF

src/common

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+#!/usr/bin/env sh
+
+INPUT_FILE="$(mktemp -t)"
+cat > "$INPUT_FILE" <&0
+export INPUT_FILE
+
+REPOSITORY="$(jq -r .source.repository < "$INPUT_FILE")"
+TAG="$(jq -r .source.tag < "$INPUT_FILE")"
+
+export REPOSITORY
+export TAG

src/in

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+#!/usr/bin/env sh
+
+set -eu
+
+. ./common
+
+oci_dir="$(mktemp -d)"
+crane pull --format=oci "$REPOSITORY:@$(jq -r .version "$INPUT_FILE")" "$oci_dir"
+pushd "$oci_dir" && tar cf "$1/image.tar" && popd
+
+echo "$(jq -r .version < "$INPUT_FILE")"

src/out

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+#!/usr/bin/env sh
+
+set -eu
+
+. ./common
+
+IMAGE_REFS_FILE="$(mktemp -t)"
+oci_dir="$(mktemp -d)"
+pushd "$oci_dir" && tar xf "$1/$(jq -r .params.image < "$INPUT_FILE")" && popd
+crane push "$1" "$REPOSITORY:$TAG" --image-refs="$IMAGE_REFS_FILE"
+sed 's/^[^@]*@//' < "$IMAGE_REFS_FILE"