{"id":23591,"date":"2025-07-12T05:34:08","date_gmt":"2025-07-12T05:34:08","guid":{"rendered":"https:\/\/www.devopsschool.com\/blog\/?p=23591"},"modified":"2025-07-12T05:38:37","modified_gmt":"2025-07-12T05:38:37","slug":"understanding-dockerfiles-instructions-options-of-docker","status":"publish","type":"post","link":"https:\/\/www.devopsschool.com\/blog\/understanding-dockerfiles-instructions-options-of-docker\/","title":{"rendered":"Docker Tutorials: Docker Image – Understanding Dockerfiles instructions & options"},"content":{"rendered":"\n

Here\u2019s a step-by-step tutorial for Dockerfile<\/strong>, including explanations and examples for each major command.<\/p>\n\n\n\n

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Dockerfile Tutorial<\/h1>\n\n\n\n

A Dockerfile<\/strong> is a text file containing instructions to build a Docker image. Each instruction creates a new layer in the image. Below are the most commonly used Dockerfile commands, their descriptions, and usage examples.<\/p>\n\n\n\n

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1. FROM<\/code><\/h2>\n\n\n\n

Description<\/strong>:
Specifies the base image to use. This must be the first command in the Dockerfile.<\/p>\n\n\n\n

Example:<\/strong><\/p>\n\n\n

FROM<\/span> ubuntu<\/span>:22.04<\/span>\n<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
This starts your build from the official Ubuntu 22.04 image.<\/p>\n\n\n\n
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2. MAINTAINER<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Optionally defines the author or maintainer of the image (deprecated in favor of LABEL).<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
MAINTAINER Rajesh Kumar <rajesh@example.com<\/span>><\/span>\n<\/code><\/span>Code language:<\/span> HTML, XML<\/span> (<\/span>xml<\/span>)<\/span><\/small><\/pre>\n\n\n
Modern alternative:<\/strong><\/p>\n\n\n
LABEL maintainer=\"Rajesh Kumar <rajesh@example.com<\/span>><\/span>\"\n<\/code><\/span>Code language:<\/span> HTML, XML<\/span> (<\/span>xml<\/span>)<\/span><\/small><\/pre>\n\n\n
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3. ADD<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Copies files\/directories from the host into the image. Can also extract local tar archives.<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
ADD myapp.tar.gz \/opt\/\nADD config\/settings.json \/etc\/myapp\/\n<\/code><\/span><\/pre>\n\n\n
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4. COPY<\/code><\/h2>\n\n\n\n
Description<\/strong>:
(Copy is very similar to ADD but doesn\u2019t do auto-extraction or remote URLs; prefer COPY unless you need ADD features.)<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
COPY . \/app\/\n<\/code><\/span><\/pre>\n\n\n
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5. RUN<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Executes commands in a new layer during build, e.g., install packages.<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
RUN apt-get<\/span> update && apt-get<\/span> install -y nginx\nRUN mkdir \/app\n<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
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6. CMD<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Sets the default command to run when a container is started from the image (can be overridden).<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
CMD<\/span> [\"nginx\"<\/span>, \"-g\"<\/span>, \"daemon off;\"<\/span>]<\/span>\n<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
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7. ENTRYPOINT<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Sets a command that will always run (even if arguments are passed to docker run<\/code>).
Allows for combining with CMD for default args.<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
ENTRYPOINT<\/span> [\"python3\"<\/span>, \"app.py\"<\/span>]<\/span>\nCMD<\/span> [\"--help\"<\/span>]<\/span>\n<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
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8. ENV<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Sets environment variables in the image.<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
ENV APP_ENV=production\nENV PATH=\"\/opt\/myapp\/bin:${PATH}\"<\/span>\n<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
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9. EXPOSE<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Documents which port(s) the app inside the container uses (does not actually publish the port).<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
EXPOSE 80\nEXPOSE 443\n<\/code><\/span><\/pre>\n\n\n
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10. USER<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Sets the default user to run commands as in the image.<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
USER www-data\n<\/code><\/span><\/pre>\n\n\n
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11. VOLUME<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Defines a mount point for external volumes or persistent data.<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
VOLUME<\/span> [\"\/data\"<\/span>]<\/span>\n<\/code><\/span>Code language:<\/span> CSS<\/span> (<\/span>css<\/span>)<\/span><\/small><\/pre>\n\n\n
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12. WORKDIR<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Sets the working directory for RUN, CMD, ENTRYPOINT, COPY, and ADD.<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
WORKDIR \/app\n<\/code><\/span><\/pre>\n\n\n
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13. ONBUILD<\/code><\/h2>\n\n\n\n
Description<\/strong>:
Adds a trigger instruction to be executed when the image is used as the base for another build.<\/p>\n\n\n\n
Example:<\/strong><\/p>\n\n\n
ONBUILD COPY . \/app\/src\n<\/code><\/span><\/pre>\n\n\n
Useful for images intended to be used as parent images (like language or framework base images).<\/p>\n\n\n\n
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Example: Complete Dockerfile<\/h1>\n\n\n
FROM python:3.11<\/span>-slim\nLABEL maintainer=\"Rajesh Kumar <rajesh@example.com>\"<\/span>\n\nWORKDIR \/app\n\nCOPY requirements.txt .\nRUN pip install -r requirements.txt\n\nCOPY . .\n\nENV FLASK_ENV=production\n\nEXPOSE 5000<\/span>\n\nCMD [\"python\"<\/span>, \"app.py\"<\/span>]\n<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
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How to Build & Run<\/h2>\n\n\n\n
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  1. Build the image:<\/strong> docker build -t my-flask-app .<\/code><\/li>\n\n\n\n
  2. Run the container:<\/strong> docker run -p 5000:5000 my-flask-app<\/code><\/li>\n<\/ol>\n\n\n\n
    \n\n\n\n
    Quick Reference Table<\/h2>\n\n\n\n
    Command<\/th>Description<\/th><\/tr><\/thead>
    FROM<\/td>Sets base image (first line)<\/td><\/tr>
    MAINTAINER<\/td>Maintainer info (deprecated, use LABEL)<\/td><\/tr>
    LABEL<\/td>Set metadata info<\/td><\/tr>
    ADD<\/td>Copy files\/dirs & extract archives<\/td><\/tr>
    COPY<\/td>Copy files\/dirs (no extract)<\/td><\/tr>
    RUN<\/td>Execute commands at build time<\/td><\/tr>
    CMD<\/td>Default command at container start<\/td><\/tr>
    ENTRYPOINT<\/td>Main entry command (always runs)<\/td><\/tr>
    ENV<\/td>Set environment variables<\/td><\/tr>
    EXPOSE<\/td>Declare ports to be exposed<\/td><\/tr>
    USER<\/td>Set default user<\/td><\/tr>
    VOLUME<\/td>Define mount point for volumes<\/td><\/tr>
    WORKDIR<\/td>Set default working directory<\/td><\/tr>
    ONBUILD<\/td>Add trigger instructions for child builds<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
    \n\n\n\n
    Summary<\/h2>\n\n\n\n