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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?

- A tool to build and run containers
- Docker engine runs containers using Linux features:
  - Namespaces
  - cgroups
  - Union file systems
- Uses images layered from base -> app code

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## Docker Architecture

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

<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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  Overlayfs is the default.  This allows Docker to use a layered approach.  In this example the bottom layer or lowerdir is the "filesytem" from the image.  The upperdir is the container filesystem (not persisted by default) and the merged is volume/bind mounts. 
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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 # ro for ReadOnly
$ docker run --volume /home/mikeconrad/projects/example/app:dst=/app
```
<!-- https://docs.docker.com/engine/storage/bind-mounts/ -->

<!-- 
These terms are oftentimes used interchangebly and can be confusing but it is important to understand the difference.
You need to be careful when using bind mounts because by default, the processes running in the container will have read/write access to your filesystem.  
This could cause some issues if code running inside the container is malicious or is compromised.  
It is also possible to mount the files as read-only so that the container has access to read them but not write.  This is better for security.  
Bind mounts also "overwrite" the container/image filesystem layers.  So for example mounting ./some-files/test:/etc/passwd would overwrite the /etc/passwd file in the container 
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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"]
```

- Starts with a base image
- Copy files and install deps
- Set default command

```bash
$ docker build -t node-app .
```
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## Multi Stage builds

```dockerfile
# Stage 1: Build the Go binary
FROM node:22-alpine AS base
FROM base AS build
# Set working directory inside the build container
WORKDIR /app
COPY package*.json ./
RUN yarn

FROM base AS develop
COPY --from=base /app/node_modules /app/node_modules
COPY . .

ENTRYPOINT ["yarn", "dev"]
EXPOSE 3000
```

- Use specific versions, not `latest`
- Combine commands to reduce layers
- Use `.dockerignore`
- Prefer slim or alpine images
- Run as non-root user if possible

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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