Sign container image using Cosign in keyless mode

Cosign keyless mode makes possible to sign the container image using the
OIDC Identity Tokens provided by GitHub Actions [0][1].
The signature is published to the registry storing the image and to the
public Rekor transparency log instance [2].

Cosign keyless mode has already been adopted by some major projects like
Kubernetes [3].

The image signature can be manually verified using:
```
$ cosign verify \
	--certificate-oidc-issuer='https://token.actions.githubusercontent.com' \
	--certificate-identity-regexp='^https://github.com/meilisearch/meilisearch/.github/workflows/publish-docker-images.yaml' \
	<image_name>
```

See #2179.
Note that a similar approach can be used to sign the release binaries.

[0] https://docs.github.com/en/actions/deployment/security-hardening-your-deployments/about-security-hardening-with-openid-connect
[1] https://docs.sigstore.dev/cosign/signing/signing_with_containers/
[2] https://docs.sigstore.dev/rekor/overview
[3] https://kubernetes.io/docs/tasks/administer-cluster/verify-signed-artifacts/#verifying-image-signatures

.github/workflows/publish-docker-images.yml

@@ -16,6 +16,8 @@ on:
 jobs:
   docker:
     runs-on: docker
+    permissions:
+      id-token: write # This is needed to use Cosign in keyless mode
     steps:
       - uses: actions/checkout@v3
 
@@ -62,6 +64,9 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
+      - name: Install cosign
+        uses: sigstore/cosign-installer@3454372f43399081ed03b604cb2d021dabca52bb # tag=v3.8.2
+
       - name: Login to Docker Hub
         uses: docker/login-action@v3
         with:
@@ -85,6 +90,7 @@ jobs:
 
       - name: Build and push
         uses: docker/build-push-action@v6
+        id: build-and-push
         with:
           push: true
           platforms: linux/amd64,linux/arm64
@@ -94,6 +100,17 @@ jobs:
             COMMIT_DATE=${{ steps.build-metadata.outputs.date }}
             GIT_TAG=${{ github.ref_name }}
 
+      - name: Sign the images with GitHub OIDC Token
+        env:
+          DIGEST: ${{ steps.build-and-push.outputs.digest }}
+          TAGS: ${{ steps.meta.outputs.tags }}
+        run: |
+          images=""
+          for tag in ${TAGS}; do
+            images+="${tag}@${DIGEST} "
+          done
+          cosign sign --yes ${images}
+
       # /!\ Don't touch this without checking with Cloud team
       - name: Send CI information to Cloud team
         # Do not send if nightly build (i.e. 'schedule' or 'workflow_dispatch' event)