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# Demystifying Docker

Mike Conrad - SCS 2025

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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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<img src="https://miro.medium.com/v2/resize:fit:400/format:webp/1*Ibnwjo9LtUFxRY1MZgOcvg.png"/>

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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).  
<br />
<br />
### Allows us to 
- Test changes in isolated environments
- Simplify complex dev environment setups
  - (frontend/backend services, databases, object storage, etc)

<!--A single VM in the cloud running `Docker Compose` with `Traefik` and `Sablier` allows us to have multiple ephemeral preview environments running at any given time.-->
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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)

<img src="https://docs.docker.com/get-started/images/docker-architecture.webp" width="700" />

[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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![UnionFS diagram](https://docs.docker.com/engine/storage/drivers/images/overlay_constructs.webp)

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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
```bash
$ 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
```
<!-- https://docs.docker.com/engine/storage/bind-mounts/ -->

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## Volume Mount Example
```bash
$ 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 
```
```bash
$ 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.
<!-- https://docs.docker.com/engine/storage/bind-mounts/ -->

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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

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

```dockerfile
# 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
```

```bash
$ 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)](https://git.hackanooga.com/mikeconrad/demystifying-docker-v2)
- [DocketProxy (Docker socket proxy)](https://git.hackanooga.com/mikeconrad/docketproxy)
- [SlimToolkit (Optimize and secure containers)](https://github.com/slimtoolkit/slim)

## 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