Docker containers operate as isolated userland environments that leverage the host machine's kernel rather than booting a separate operating system. This architectural choice eliminates the overhead of traditional virtualization while maintaining process and filesystem isolation through Linux namespaces and cgroups. Each container relies on a base image that supplies the foundational user-space components, such as system libraries, package managers, and shell utilities, which collectively simulate a complete operating system environment.

The selection of this base layer determines the available package repositories, security update cycles, and runtime compatibility. Engineers specify the foundational layer using the FROM directive within a build configuration file. Popular distributions like Debian, Alpine, and RHEL derivatives are commonly adapted into stripped-down images optimized for containerized workloads.

# Build configuration for a lightweight runtime environment
FROM debian:stable-slim

ENV LISTEN_PORT=3000
ENV WORKSPACE=/opt/service

RUN apt-get update && \
    apt-get install -y --no-install-recommends curl ca-certificates && \
    curl -fsSL https://deb.nodesource.com/setup_18.x | bash - && \
    apt-get install -y nodejs && \
    rm -rf /var/lib/apt/lists/*

WORKDIR ${WORKSPACE}

COPY init.sh /usr/local/bin/
RUN chmod +x /usr/local/bin/init.sh

EXPOSE ${LISTEN_PORT}

ENTRYPOINT ["/usr/local/bin/init.sh"]
CMD ["node", "app.js"]

The configuration above establishes a Debian-based runtime with Node.js installed via an external repository. Environment variables dictate the network port and directory path, while package installation commands are chained and cache directories are cleared to minimize the final image footprint. The ENTRYPOINT and CMD instructions are decoupled, allowing an initialization script to handle prerequisite checks before delegating control to the primary application process.

Container build systems interpret these layered definitions to construct immutable artifacts. During the image creation phase, each instruction generates a new read-only layer. These layers are then stacked into a unified overlay filesystem when the container launches. This mechanism ensures consistent execution environments across disparate infrastructure without the resource penalties of hardware-level virtualization.