Docker solves the "works on my machine" problem by providing lightweight, fast containers that ensure environment parity across development, staging, and production. Unlike traditional local installations or VMs, containers package applications with minimal operating systems (as small as 5MB), start instantly, and allow running multiple versions of tools like PHP without conflicts. Dockerfiles enable building custom images with specific dependencies, which can be shared via registries to save compilation time. This approach eliminates inconsistent environments, simplifies onboarding, and ensures every developer works with identical setups.