Docker Internals

Docker is a packaging and runtime system for containers. A container looks like a small isolated machine, but it is not a full virtual machine. It is still a normal process running on the host kernel. Docker makes that process portable by combining filesystem layers, resource controls, network plumbing, and a standard image format.

The visible pieces are the Docker client, the Docker daemon, and a registry. The client is the docker command you type. It talks to the daemon over a local socket or remote API. The daemon manages images, containers, networks, and volumes. Registries such as Docker Hub or a private registry store versioned images so hosts can pull the same build artefact.

An image is a read-only package built in layers. Each layer represents a filesystem change, such as adding a binary, installing dependencies, or copying application code. Layers are content addressed, which means they are identified by a digest derived from their contents. If two images share the same base layers, Docker can reuse those layers instead of storing duplicate copies. This is why image order in a Dockerfile matters. Stable steps near the top improve cache reuse. Frequently changing steps near the top invalidate everything below them and make builds slower.

When you run docker build, modern Docker usually relies on BuildKit. It reads the Dockerfile, executes each step in an isolated build environment, snapshots the resulting filesystem changes, and emits an OCI compatible image manifest plus layers. The finished image can stay on the local host or be pushed to a registry.

When you run docker pull, the daemon contacts the registry, fetches the image manifest, then downloads only the layers missing from the host. Those layers are unpacked into the local image store. Pulling is efficient because unchanged layers are reused across tags and containers.

When you run docker run, Docker combines an image with runtime configuration and creates a container. Under the hood, the container runtime sets up Linux namespaces so the process sees its own process tree, network stack, mount table, and hostname. Control groups, or cgroups, limit and account for CPU, memory, and other resources. A union filesystem such as OverlayFS presents the image layers as one filesystem and adds a thin writable layer on top. Writes go into that top layer, which is why container changes disappear when the container is removed unless you attach a volume.

Networking is another important part. By default Docker often connects the container to a bridge network and gives it a private IP address. Port publishing creates rules so traffic from the host can reach the container. That indirection is convenient, but it also means container networking failures often come from NAT rules, DNS settings, service binding, or conflicting port mappings rather than the application itself.

Containers are lightweight compared with virtual machines, but they are not a security boundary you should overstate. A container shares the host kernel. If the process runs as root with broad capabilities, a kernel flaw or bad mount can have host impact. Minimal images, dropped capabilities, read-only filesystems, non-root users, and strict secret handling all matter in real deployments.

So Docker works by turning an application into layered images, distributing those images through registries, and launching them as isolated host processes with controlled filesystems, networking, and resources. The abstraction feels simple, but the operational details still matter. Fast builds depend on layer discipline, reliable state depends on volumes, and safe production use depends on treating containers as well-contained processes rather than tiny virtual servers.