Docker lets you have a standard easy to run reproducible description of a system (service + all dependencies) as code for any service. It's a fat-binary for any

Docker lets you have a standard easy to run reproducible description of a system (service + all dependencies) as code for any service. It's a fat-binary for anything that is also the build env for that binary. Not all languages support native fat-binaries and even those that do you still need a way to reproducibly build that binary (CI doesn't count because local testing builds can't reproduce it).