Docker Fundamentals
Docker is a platform for building, shipping, and running applications in containers. It packages software with its dependencies into images, distributes them through registries, and executes them in isolated environments that behave consistently from a developer's laptop to production servers.
itVirtualization, containers, and orchestration | OpenSkills.info
Intro
Docker Fundamentals
Docker gives you a consistent way to package and run an application. You put the application and its runtime dependencies into an image. Docker starts an isolated process from that image as a container.
This model solves a common delivery problem. An application may work on one machine and fail on another because the operating system, libraries, files, or configuration differ. An image records the filesystem and default runtime configuration that the application expects. You can move that image through development, testing, and deployment without rebuilding the environment by hand.
Docker is more than the container process. The Docker client sends requests to Docker Engine. Docker Engine builds images, manages local objects, and starts containers. A registry stores images so other machines can pull them. Docker Compose describes a multi-container application in one YAML file.
The central workflow is short:
Write a Dockerfile → build an image → push or pull through a registry → run a container
Everything else supports that flow.
Why containers are useful
A container packages an application while still using the host operating system kernel. It does not boot a complete guest operating system for each workload. Docker uses operating-system features such as namespaces and control groups to isolate processes and account for resources.
That makes containers useful for repeatable application environments, local development, automated testing, service delivery, and short-lived jobs. You can replace a container from the same image instead of repairing its filesystem by hand.
Containers do not remove differences among processor architectures or operating-system kernels. An image must support the target platform. Linux and Windows containers also require compatible host capabilities. Treat an image as a repeatable application package, not as a universal machine snapshot.
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Sources
- https://docs.docker.com/get-started/docker-overview/
Supports
- Docker client and daemon architecture
- Docker objects including images, containers, networks, and volumes
- Images as read-only templates and containers as runnable image instances
- Registry pull and push workflow and Docker Hub as the default public registry
- Namespaces and control groups as container isolation and resource mechanisms
- https://docs.docker.com/get-started/
Supports
- Official foundational learning path for installing Docker and learning its concepts
- Docker concepts and workshop as next steps for new users
- https://docs.docker.com/get-started/docker-concepts/
Supports
- Guided foundation covering images, containers, registries, ports, storage, and sharing applications
- https://docs.docker.com/get-started/docker-concepts/the-basics/what-is-an-image/
Supports
- Images as standardized packages of files, binaries, libraries, and configuration
- Image immutability and layered composition
- Base images and image selection from a registry
- https://docs.docker.com/reference/dockerfile/
Supports
- Dockerfile purpose and supported instructions
- Roles of FROM, COPY, RUN, WORKDIR, ENV, USER, EXPOSE, ENTRYPOINT, and CMD
- EXPOSE as port metadata rather than automatic port publishing
- https://docs.docker.com/build/cache/
Supports
- Image instructions producing ordered layers
- Reuse of unchanged build results
- Rebuilding downstream layers after a layer changes
- https://docs.docker.com/build/cache/invalidation/
Supports
- Cache checks for Dockerfile instructions and copied files
- Cache invalidation after changed ADD and COPY inputs
- Ordering stable work before frequently changed work
- https://docs.docker.com/engine/storage/
Supports
- Writable container layer lifecycle
- Volume, bind mount, tmpfs, and named-pipe storage choices
- Bind mounts linking host paths and containers
- tmpfs data living in host memory without disk persistence
- https://docs.docker.com/engine/storage/volumes/
Supports
- Volumes as Docker-managed persistent data stores
- Volume lifecycle independent from a container
- Volume and bind-mount decision guidance
- Named volume persistence after container removal
- https://docs.docker.com/engine/network/
Supports
- Default outgoing container connectivity
- User-defined networks and container-name communication
- Network attachment and published-port behavior
- Built-in network drivers and network isolation choices
- https://docs.docker.com/get-started/docker-concepts/running-containers/publishing-ports/
Supports
- Host-to-container port forwarding syntax and behavior
- Published ports binding to all host interfaces by default
- EXPOSE not publishing a port by itself
- https://docs.docker.com/compose/
Supports
- Compose as a tool for defining and running multi-container applications
- Services, networks, and volumes in one YAML configuration
- Lifecycle commands for a Compose application
- https://docs.docker.com/compose/intro/compose-application-model/
Supports
- Compose file and CLI relationship
- Services as application components connected through networks
- Compose application resources and project grouping
- https://docs.docker.com/engine/security/
Supports
- Docker Engine security areas including namespaces, control groups, daemon exposure, capabilities, and kernel hardening
- Root privileges of the standard daemon unless rootless mode is selected
- Resource accounting and limiting through control groups
- https://docs.docker.com/engine/security/rootless/
Supports
- Rootless daemon and containers running without root privileges
- User-namespace implementation and privilege-reduction purpose
- Rootless mode prerequisites and operational considerations