demystifying-docker/slides.md

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Demystifying Docker

Mike Conrad - SCS 2025

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Why Containers?

• "It works on my machine" is a thing of the past
• Containers are lightweight and portable
• "Boot" in milliseconds
• Ideal for reproducible dev environments

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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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Common Use Cases

• Reproducible Dev environments (dev containers)
• Preview/PR environments (ephemeral test environments)
• Legacy applications or applications with complex environment setups

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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)

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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 --from=install-deps /app/node_modules /app/node_modules
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

• Slide Deck (including examples)
• DocketProxy (Docker socket proxy)
• SlimToolkit (Optimize and secure containers)

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