Fix resolution of bootstrapper docker tag in release build.

[td202]

Purpose

Change to look for the docker tag under docker-bootstrapper instead of docker-p2p-bootstrapper. This is to match:

  DOCKER_TAGS_TEMPLATES: '{
      \"docker-stagenet\": \"concordium/stagenet-node:${VERSION}\",
      \"docker-testnet\": \"concordium/testnet-node:${VERSION}\",
      \"docker-mainnet\": \"concordium/mainnet-node:${VERSION}\",
      \"docker-bootstrapper\": \"concordium/bootstrapper:${VERSION}\"
    }'