Docker is an open source platform for developing, shipping, and running applications. It enables the separation of applications from infrastructure by packaging software into standardized units called containers that include everything needed to run: code, runtime, system tools, system libraries, and settings.
Docker changed software delivery by enforcing consistency across environments (Dev, Stage, Prod). It reduced dependency issues and enabled the microservices architecture pattern by making the deployment unit smaller and immutable.
Docker made containers accessible, but "Docker" the tool is distinct from the OCI (Open Container Initiative) standards. We value the OCI image format as the universal shipping manifest for our software. Whether the runtime is Docker, containerd, or Podman is an implementation detail; the immutability of the artifact is the value.