container build improvements

[aceat64]

• build otfd and otf-agent images from the same Dockerfile
  • Consistent image builds
• compile binaries in docker build
  • Simplifies the build process, as all build-time dependencies are handled within Docker
• docker build cache optimizations
  • binaries are only built once (per platform) and then copied into the final images
  • leverages Docker cache volumes, without leaving any cache files in the image itself
• run container images as non-root user/group
  • Improved security
• tini for signal handling and reaping zombie processes
  • More info: Why Tini?