Tutorials for Building Docker Containers From Scratch: 3 Step-by-Step Guides

The codecrafters-io/build-your-own-x repository hosts a dedicated "Build your own Docker" section in its README.md that links to three hands-on tutorials—written in C, Go, and Python—teaching you how to implement Linux namespaces, cgroups, and filesystem isolation to create container runtimes from the ground up.

The codecrafters-io/build-your-own-x repository curates tutorials for building Docker containers from scratch, providing hands-on guides for recreating containerization technologies. These resources teach you to construct working runtimes without relying on Docker's existing binaries, covering the exact kernel features—such as CLONE_NEWUTS, CLONE_NEWPID, and chroot—that power modern container engines.

Where to Find Tutorials for Building Docker Containers From Scratch

The tutorials are indexed in the repository's main README.md file under the "Build your own Docker" heading. This section aggregates community-written guides that each implement a minimal container runtime in a different programming language, allowing you to choose the implementation that best matches your preferred tech stack.

The Three Core Tutorials

Each tutorial demonstrates how to construct a container runtime using raw Linux system calls and kernel features:

• C: Linux containers in 500 LOC – This guide demonstrates creating a new process with unshare, isolating it with namespaces, and running a shell inside a chroot environment using fewer than 500 lines of C code.

• Go: Build Your Own Container Using Less Than 100 Lines of Go – This concise implementation sets up CLONE_NEWUTS, CLONE_NEWPID, and CLONE_NEWNS namespaces, mounts a minimal filesystem, and executes user commands via syscall.Exec.

• Python: Rebuild Docker from Scratch – A workshop-style tutorial that provides a verbose, step-by-step Python implementation showing image extraction, layering mechanics, and a simple command-line interface for container management.

Core Container Concepts Covered

All three tutorials walk through the fundamental primitives required for containerization:

Namespace Isolation

The guides implement Linux namespaces to isolate processes from the host system. You will configure CLONE_NEWUTS for hostname isolation, CLONE_NEWPID for process ID separation, CLONE_NEWNS for mount namespaces, and optionally CLONE_NEWNET for network isolation.

Filesystem Isolation

Each tutorial demonstrates preparing a root filesystem using chroot or pivot_root to switch to an isolated filesystem tree. This prevents the containerized process from accessing host system files, mirroring Docker's filesystem isolation model.

Process Execution and Resource Limits

The implementations use system-specific exec functions—execve in C, syscall.Exec in Go, and os.execve in Python—to replace the container init process with the user's target command. The Go tutorial additionally covers cgroups for resource limiting, though this is optional in the other guides.

Practical Implementation: Minimal Go Container

Below is a self-contained Go program based on the "Build Your Own Container Using < 100 Lines of Go" tutorial. This implementation creates new namespaces, mounts a proc filesystem, changes root to an isolated filesystem, and executes the provided command:

package main

import (
	"log"
	"os"
	"os/exec"
	"syscall"
)

func main() {
	if len(os.Args) < 2 {
		log.Fatalf("usage: %s <command> [args...]", os.Args[0])
	}
	cmd := os.Args[1]
	args := os.Args[2:]

	// 1. Reexec the current binary in new namespaces
	cmdPath, err := exec.LookPath(os.Args[0])
	if err != nil {
		log.Fatalf("cannot find self binary: %v", err)
	}
	// The child will notice the flag `-child`
	if os.Getenv("GO_CONTAINER_CHILD") == "" {
		attr := &syscall.SysProcAttr{
			Cloneflags: syscall.CLONE_NEWUTS |
				syscall.CLONE_NEWPID |
				syscall.CLONE_NEWNS,
			Unshareflags: syscall.CLONE_NEWNS,
		}
		childEnv := append(os.Environ(), "GO_CONTAINER_CHILD=1")
		if err := syscall.Exec(cmdPath, os.Args, childEnv); err != nil {
			log.Fatalf("failed to re‑exec: %v", err)
		}
	}

	// 2. Inside the child: mount proc, set hostname, chroot
	if err := syscall.Sethostname([]byte("container")); err != nil {
		log.Fatalf("sethostname: %v", err)
	}
	rootfs := "/rootfs" // <-- mount an Alpine rootfs here
	if err := syscall.Chroot(rootfs); err != nil {
		log.Fatalf("chroot: %v", err)
	}
	if err := os.Chdir("/"); err != nil {
		log.Fatalf("chdir: %v", err)
	}
	if err := syscall.Mount("proc", "/proc", "proc", 0, ""); err != nil {
		log.Fatalf("mount proc: %v", err)
	}

	// 3. Finally exec the desired command inside the container
	if err := syscall.Exec(cmd, append([]string{cmd}, args...), os.Environ()); err != nil {
		log.Fatalf("exec %s: %v", cmd, err)
	}
}

Running the Container

To execute this minimal container runtime:

# Prepare a root filesystem (e.g., extract Alpine Linux)

mkdir -p /rootfs && tar -C /rootfs -xvf alpine.tar

# Build the binary

go build -o tinycontainer main.go

# Run a shell inside the isolated environment (requires root for namespaces)

sudo ./tinycontainer /bin/sh

The resulting binary creates an isolated environment with its own PID 1, distinct hostname (container), and restricted filesystem view—functionally equivalent to Docker's isolation model without requiring the Docker daemon.

Summary

• The codecrafters-io/build-your-own-x repository maintains tutorials for building Docker containers from scratch in C, Go, and Python.
• All tutorials implement core Linux container primitives: namespaces (CLONE_NEWUTS, CLONE_NEWPID, CLONE_NEWNS), chroot filesystem isolation, and direct process execution via system calls.
• The Go implementation provides the most concise example at under 100 lines, while the Python workshop offers the most detailed explanation of image layering and container lifecycle management.
• Each tutorial produces a runnable binary that can launch containerized processes without any Docker-specific dependencies.

Frequently Asked Questions

Do I need Docker installed to follow these tutorials?

No. These tutorials teach you how to build container runtimes using only standard Linux kernel features and system calls. You only need a Linux environment with root access to create namespaces and mount filesystems, plus the respective compiler or interpreter for your chosen language (GCC for C, Go toolchain, or Python).

Which programming language is best for learning containers from scratch?

Go offers the most concise implementation at under 100 lines and handles system calls cleanly, making it ideal for understanding the core mechanics quickly. However, Python provides the most verbose, educational walkthrough of advanced concepts like image layering, while C offers the closest view of raw kernel interfaces without runtime abstractions.

How do these tutorials handle container networking?

The primary tutorials focus on process and filesystem isolation using CLONE_NEWNET for basic network namespace creation. While they establish separate network stacks, they typically use simple loopback or host networking defaults. Advanced networking features like virtual Ethernet pairs (veth), bridges, or port mapping require additional implementation beyond the core tutorials.

Can I use this code in production environments?

No. These implementations are educational tools designed to demonstrate container principles. They lack critical production features such as comprehensive security sandboxing, image verification, robust error handling, and management APIs. Use these tutorials to learn kernel concepts, then rely on established container runtimes like containerd or Docker for production workloads.

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