Trailing newline included in variable assignment after grep, and sent to GitHub actions output – Ubuntu

I’ve got a bash script in a GitHub action that greps for image tags, then replaces them in a docker-compose.yml.

# docker-compose.yml from a JSON payload
docker_compose='"services:n  api:n    proxy: ghcr.io/org/api:main-4095094301n    ports:n      - 0:80n    restart: alwaysnn  proxy:n    image: ghcr.io/org/proxy:main-4095124301n    depends_on:n      - apin    environment:n      - "ENVIRONMENT=dev"n      - "PORT=8000"n    ports:n      - 0:3000n    restart: alwaysnn ..."'

# Extract the tag of each image, send to GitHub actions output
echo "proxy_tag=$(echo -n $docker_compose | grep -oP 'proxy:K[-w]+')" >> $GITHUB_OUTPUT

If I remove the >> operator to echo the output, everything looks OK:

proxy_tag=main-4095094301

But when I feed it into sed later in a different step of the pipeline, an extra newline character seems to come from nowhere:

echo "running s/PROXY_TAG/$proxy_tag/"
sed -i "s/PROXY_TAG/$proxy_tag/" docker-compose.yml

# running s/PROXY_TAG/main-4095094301
# /
# sed: -e expression #1, char 18: unterminated `s' command
# Error: Process completed with exit code 1.

I’ve tried some common suggestions eg. piping output through tr -d 'n':

echo "proxy_tag=$(echo -n $docker_compose | grep -oP 'proxy:K[-w]+' | tr -d 'n')" >> $GITHUB_OUTPUT

Is there something missing I don’t understand about bash vars or Github actions?

See below a more complete context of how where these commands are being used.

name: Update Docker Compose

on:
  workflow_dispatch:
    inputs:
      proxy_tag:
        description: proxy_tag
        type: string
        required: false
      # api_tag:
      # site_tag:

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v3

      - name: Fetch docker-compose
        id: get-tags
        run: |
            docker_compose=$(curl ...)

            echo "proxy_tag=$(echo -n $docker_compose | grep -oP 'proxy:K[-w]+')" >> $GITHUB_OUTPUT
            echo "api_tag=$(echo $docker_compose | grep -oP 'api:K[-w]+')" >> $GITHUB_OUTPUT
            echo "site_tag=$(echo $docker_compose | grep -oP 'site:K[-w]+')" >> $GITHUB_OUTPUT

      - name: Replace tags docker-compose
        env:
          proxy_tag: >
            ${{ github.event_name == 'workflow_dispatch'
                && github.event.inputs.proxy_tag
              || steps.get-tags.outputs.proxy_tag
              || 'latest'
            }}
          # api_tag: >
          # site_tag: >
        run: |
          echo "Setting [$proxy_tag]"

          sed -i "s/PROXY_TAG/$proxy_tag/" docker-compose.yml
          sed -i "s/API_TAG/$api_tag/" docker-compose.yml
          sed -i "s/SITE_TAG/$site_tag/" docker-compose.yml

        # Outputs:
        #   Setting [main-4095094301       # <--- notice the newline
        #   ]
        #
        #   sed: -e expression #1, char 18: unterminated `s' command
        #   Error: Process completed with exit code 1.