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seriph https://cover.sli.dev Welcome to Slidev ## Slidev Starter Template Presentation slides for developers. Learn more at [Sli.dev](https://sli.dev) text-center
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Demystifying Docker

Mike Conrad - SCS 2025

Press Space for next page

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Who is this for?

About you

  • Some experience with Docker/containers
  • Some experience with BASH
  • Want to better understand how containers work

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Follow Along

Example Repo - https://git.hackanooga.com/mikeconrad/demystifying-docker

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Common Use cases for containers

  • Reproducible dev environments
  • Testing in CI/CD environments
  • Better "Portability" of application code
  • Snapshot of application code at specific point in time

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How we use containers

  • PR builds (Preview Environments).


Allows us to

  • Test changes in isolated environments
  • Simplify complex dev environment setups
    • (frontend/backend services, databases, object storage, etc)

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Containers vs Virtual Machines

Feature VM Container
Boot time Minutes Seconds
Resource usage Heavy Lightweight
Isolation Strong Process-level
Portability Medium Very High

In reality we use containers and vm's together. Containers run inside of VM's for better security and isolation, especially in cloud and multi tenant environments.

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What is Docker?

  • Written in GO
  • Uses Client/Server model with REST API (docker cli and dockerd)
  • Eco system of tools (Compose, Swarm, etc)
  • Public Image Registry (Dockerhub)
  • Docker client typically runs on same machine as server but doesn't have to

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What is Docker?

  • A tool to build and run containers
  • Containers are exclusive to Linux
  • Docker engine runs containers using Linux features like:
    • Namespaces
    • cgroups
    • Union file systems
  • Container runs from an image layered with base image and application code

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Docker Architecture

Docker CLI (Client) <-- REST API --> Docker Engine (Server)

[https://docs.docker.com/get-started/docker-overview/]

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Docker Under the Hood

  • Namespaces: isolate PID, net, mount, etc.
  • cgroups: control CPU, memory, IO
  • UnionFS: layered filesystem (OverlayFS)

UnionFS diagram

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Bind/Volume Mounts

  • 2 most common storage mechanisms
  • Different use cases and security implications

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Bind Mounts

  • Mounting files/directories from the host machine directly into a container (merged overlayfs layer).
  • Processes inside container can modify files on host system.
  • Bind mounts are strongly tied to the host
  • Best for things like dev containers where you need to mount source code into container and have hot reload, etc.

Bind Mount Example

$ docker run --mount type=bind,src=/home/mikeconrad/projects/example/app,dst=/app,ro nginx # ro for ReadOnly
$ docker run --volume /home/mikeconrad/projects/example/app:/app nginx

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Volume Mount Example

$ docker run --name postgrestest \
   --mount type=volume,src=postgresData,dst=/var/lib/postgresql/data \
   -e POSTGRES_PASSWORD=postgres \ 
   --rm postgres:16 

$ docker run --name postgrestest \
    --volume postgresData:/var/lib/postgresql/data \
    -e POSTGRES_PASSWORD=postgres \
    --rm postgres:16 

$ docker volume inspect postgresData
[
    {
        "CreatedAt": "2025-06-08T10:39:12-04:00",
        "Driver": "local",
        "Labels": null,
        "Mountpoint": "/var/lib/docker/volumes/postgresData/_data",
        "Name": "postgresData",
        "Options": null,
        "Scope": "local"
    }
]
  • Docker creates a volume named postgresData and mounts that directory inside the container.

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Volume mounts

  • Created and managed by the Docker Daemon
  • Volume data is stored on host filesystem but managed by Docker.
  • Used for persistent data.

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Anatomy of a Dockerfile

FROM node:22-slim
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

mikeconrad@pop-os:~/projects/demystifying-docker/examples/react
$ docker build -t react-app .
  • Starts with a base image
  • Copy files and install deps
  • Set default command

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Multi Stage builds

# Stage 1 - Define Base image
FROM node:22-alpine AS base
# Stage 2 Install dependencies
FROM base AS install-deps
WORKDIR /app
COPY package*.json /app/
RUN yarn
# Stage 3 Development
FROM install-deps AS develop
WORKDIR /app
COPY . .
ENTRYPOINT ["yarn", "dev", "--host=0.0.0.0"]
EXPOSE 5173

$ docker build -t react .
$ docker run --rm -P react

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What is Docker Compose?

  • Define multi-container apps in one file
  • Great for local dev and staging (and production!)

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Q/A

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Resources

VSCode plugins

https://marketplace.visualstudio.com/items?itemName=ms-azuretools.vscode-docker

https://marketplace.visualstudio.com/items?itemName=ms-vscode-remote.remote-containers

https://marketplace.visualstudio.com/items?itemName=ms-azuretools.vscode-containers