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Day One: Your App Has to Become a Container🔗

Something changed. Maybe your manager said the word "container" in a meeting and looked at you. Maybe a platform team sent an email with a deadline and the word "VMs" in the subject line. Either way, the app is no longer allowed to just run — it has to become a container, and that's now your problem.

This section gets you there. It won't make you a Docker expert or a microservices architect by the end; that's what Essentials and the tiers beyond it are for. It will get your first container built, running, and out the door.

flowchart TD
    WIC["What Is a Container, Really?"]
    GDR["Getting Docker Running<br/>on Your Machine"]
    A["Containerizing a Single App<br/>Dockerfile / build and run / debug / share"]
    B["Breaking Up a Monolith<br/>why / seams / first slice / mixed state"]
    E["Essentials<br/>(both paths converge)"]

    WIC --> GDR
    GDR -->|"you own one app"| A
    GDR -->|"you own a monolith"| B
    A --> E
    B --> E

    style WIC fill:#d97706,stroke:#cbd5e0,stroke-width:2px,color:#fff
    style GDR fill:#d97706,stroke:#cbd5e0,stroke-width:2px,color:#fff
    style A fill:#2d3748,stroke:#cbd5e0,stroke-width:2px,color:#fff
    style B fill:#2d3748,stroke:#cbd5e0,stroke-width:2px,color:#fff
    style E fill:#1a202c,stroke:#cbd5e0,stroke-width:2px,color:#fff

Why You're Here🔗

You wrote something (a web app, an API, a script that processes data on a schedule) and now it needs to run as a container so a team, a CI pipeline, or a platform can pick it up.

  • You're packaging code you already understand; the new part is the packaging
  • The end state is one Dockerfile, one image, one container
  • Containerizing a Single App covers this path.

You've got one big application, maybe one you wrote, maybe one you didn't, and someone's decided it can no longer live on a VM. That decision usually comes with an unspoken second one: it has to start looking like more than one thing.

  • The hard part isn't Docker syntax — it's deciding where the seams are
  • You'll containerize the whole thing before you split anything, then peel pieces off deliberately
  • Breaking Up a Monolith covers this path.

What Is a Container, Really?🔗

Both paths start in the same place. Before you write a Dockerfile or draw a line through a monolith, you need the actual mental model: not "a container is a lightweight VM," which is the wrong analogy repeated so often it's practically folklore.

Start here: What Is a Container, Really?

Then, before either path gets moving, make sure the tooling itself actually works: Getting Docker Running on Your Machine covers Linux, macOS, and Windows — command line only, no Docker Desktop. Skip it if docker run hello-world already works for you.

Containerizing a Single App🔗

Breaking Up a Monolith🔗

  • Why the Monolith Has to Move

    What "get off the VMs" actually means, and why lifting the whole app into one giant container only buys you time, not the outcome anyone wants.

  • Finding the Seams: How to Scope a Microservice

    Not every function is a service waiting to happen. How to spot the boundaries that are actually there versus the ones you'd be inventing.

  • Containerizing the First Slice

    Pick the least risky piece, cut it loose, and containerize it: proof that the approach works before you bet the rest of the monolith on it.

  • Living With a Partial Migration

    Six months from now you'll still have a monolith and a handful of services talking to it. That's not failure — here's how to run that state safely.

What's Next🔗

Read What Is a Container, Really? first regardless of which path you're on, then confirm docker actually works with Getting Docker Running on Your Machine, then follow the pathway that matches your situation.