Add DockerHub -> GHCR sync workflow

.github/workflows/sync.yml

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+name: Sync DockerHub images with GHCR 🔁
+
+## This workflow is a temporal solution for having all our docker images synced to GHCR. However,
+## as soon as all the repo's CI is migrated to GitHub Actions, this can be removed.
+##
+## Project tracking the progress: https://github.com/orgs/jellyfin/projects/31
+
+on:
+  schedule:
+    - cron: "0 0 * * *"
+  workflow_dispatch:
+
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    name: Image sync 🔁
+    # Errors will probably be caused by excesses in API quota, so we can safely continue.
+    # Remaining workflows will be removed in the next scheduled run.
+    continue-on-error: true 
+    strategy:
+      fail-fast: false
+      matrix:
+        image:
+          - 'jellyfin/jellyfin'
+
+    steps:
+      - name: Clone repository
+      - uses: actions/checkout@v2.3.4
+
+      # The 'echo' command masks the environment variable
+      - name: Prepare environment
+        run: chmod +x sync_image_ghcr.sh && echo "::add-mask::${GITHUB_TOKEN}"
+
+      ## Logging in to DockerHub allows for more pulls without hitting DockerHub's quota (100 vs 200).
+      - name: Login to Docker Hub
+        uses: docker/login-action@v1.10.0
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - name: Login to GitHub Container Registry
+        uses: docker/login-action@v1.10.0
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.JF_BOT_TOKEN }}
+
+      - name: Run syncing script
+        run: ./sync_image_ghcr.sh ${{ matrix.image }}
+        env:
+          GITHUB_TOKEN: ${{ secrets.JF_BOT_TOKEN }}

sync_image_ghcr.sh

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+#!/bin/bash
+set -e
+# Simple script that pushes missing/outdated tags from an image in DockerHub to GHCR, skipping up-to-date tags 
+# GitHub package registry. You need to be logged in using 'docker login' first: https://docs.github.com/es/packages/working-with-a-github-packages-registry/working-with-the-container-registry
+target_repo="ghcr.io"
+tag_file="tag_list.txt"
+original_image="$1"
+rm -rf $tag_file
+url="https://hub.docker.com/v2/repositories/${original_image}/tags"
+tag_count=$(wget -q "$url" -O - | jq -r '.count')
+
+echo "Fetching tags from DockerHub..."
+while true; do
+	results=$(wget -q "$url" -O - | jq -r '.')
+	url=$(echo "$results" | jq -r '.next')
+	echo "$results" | jq -r '.results[] | {name: .name, last_pushed: .tag_last_pushed, digests: [.images[].digest]}' >> $tag_file
+	if [ "${url}" = "null" ]
+	then
+		break
+	else
+		continue
+	fi
+done;
+unset results, url
+
+sorted=$(cat "$tag_file" | jq -s 'sort_by(.last_pushed)')
+echo "$sorted" > $tag_file
+file_tag_count=$(jq length "$tag_file")
+
+if [ $tag_count = $file_tag_count ]
+then
+	echo -e "All the data was retrieved correctly. Pushing missing/modified tags to DockerHub...\n"
+else
+	echo "The retrieved data doesn't match the amount of tags expected by Docker API. Exiting script..."
+	exit 1
+fi
+
+unset sorted, file_tag_count, tag_count
+
+## This token is that GitHub provides is used to access the registry in read-only, so users are able to
+## use GHCR without signing up to GitHub. By using this token for checking for the published images, we don't consume
+## our own API quota.
+dest_token=$(wget -q https://${target_repo}/token\?scope\="repository:${original_image}:pull" -O - | jq -r '.token')
+tag_names=$(cat "$tag_file" | jq -r '.[] | .name')
+
+while read -r line; do
+	tag="$line"
+	source_digests=$(cat "$tag_file" | jq -r --arg TAG_NAME "$tag" '.[] | select(.name == $TAG_NAME) | .digests | sort | .[]' | cat)
+	target_manifest=$(wget --header="Authorization: Bearer ${dest_token}" -q https://${target_repo}/v2/${original_image}/manifests/${tag} -O - | cat)
+	target_digests=$(echo "$target_manifest" | jq '.manifests | .[] | .digest' | jq -s '. | sort' | jq -r '.[]' | cat)
+	if [ "$source_digests" = "$target_digests" ]
+	then
+		echo The tag $tag is fully updated in $target_repo
+		continue
+	else
+		echo Updating $tag in $target_repo
+		docker pull $original_image:$tag
+		docker tag $original_image:$tag $target_repo/$original_image:$tag
+		docker push $target_repo/$original_image:$tag
+
+		# Delete pushed images from local system
+		docker image rm $original_image:$tag
+		docker image rm $target_repo/$original_image:$tag
+	fi
+done <<< $tag_names
+
+rm -rf $tag_file
+echo -e "\nAll the tags have been updated successfully"
+exit 0