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Writing a Dockerfile

The instructions FROM, WORKDIR, COPY, ADD, RUN, ENV, ARG, EXPOSE, USER, CMD and ENTRYPOINT; the build context and .dockerignore; one layer per instruction, the build cache and docker build -t.

  • 30 cards
  • 6 topics
  • Free
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The outline, topic by topic

Each topic is weighted the way the exam weighs it.

  1. Concepts
  2. Questions
  3. Sort and link
  4. The steps
  5. Scenarios
  6. Understand

A few cards from the deck

  • Dockerfile: what is it?

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    A text file that describes, instruction by instruction, how to build an image from a base image.

    It makes the build reproducible and versionable: whoever runs docker build gets the same image, instead of depending on the steps of whoever made it.

  • What is the difference between COPY and ADD?

    • ADD can also unpack a local archive and download a URL
    • COPY only copies single files, whereas ADD also copies folders
    • COPY runs when the container starts, ADD when the image is built
    • ADD adds a file to the image leaving the original in the context, COPY moves it
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    ADD can also unpack a local archive and download a URL

    Both copy from the context into the image, folders included. ADD has two extra behaviours, sometimes surprising; COPY is preferred when you only want to copy.

  • The … instruction runs when the image is built, whereas … defines the command run when the container starts.

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    The RUN instruction runs when the image is built, whereas CMD defines the command run when the container starts.

    RUN npm ci installs the dependencies once and for all in a layer; CMD ["node", "server.js"] only runs at docker run.

  • Image layer: what is it?

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    The set of file-system changes produced by one build instruction, stacked on the previous ones.

    RUN, COPY and ADD instructions each create a layer. Layers are read-only and shared between images, which saves downloads and disk.

  • A Node Dockerfile that benefits from the cache: which step comes after "FROM node:24-alpine"?

    • WORKDIR /app
    • COPY package*.json ./
    • RUN npm ci
    • COPY . .
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    WORKDIR /app

    The base image first, then the working folder, then what rarely changes (dependencies), then what often changes (the code), and the start command last.

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