Best Practices for Writing Efficient Dockerfiles

Using a Dockerfile to build images offers many benefits, including easy version control, clear build process traceability, and consistent results across environments, but improper use can lead to long build times, failures, and oversized images; this lesson teaches how to write optimal Dockerfiles for production.

Dockerfile Writing Principles

(1) Single Responsibility : Each container should run a single process, so different functionalities are split into separate containers.

(2) Provide Comments : Treat the Dockerfile as code, add comments to make each instruction’s purpose clear for collaborators.

(3) Keep Containers Minimal : Avoid installing unnecessary packages (e.g., vim, gcc) to speed up builds and reduce image size.

(4) Choose an Appropriate Base Image : Use the smallest base that satisfies the application’s runtime requirements (e.g., JRE instead of JDK for Java apps).

(5) Use a .dockerignore File : Similar to .gitignore, it excludes files that are not needed for the build, improving efficiency.

(6) Leverage Build Cache : Docker creates a layer for each instruction; placing rarely‑changed instructions early increases cache hit probability, while frequently‑changed steps (e.g., copying source code) should be placed later.

(7) Set the Correct Timezone : Adjust the container’s timezone to match the desired region (e.g., China Standard Time).

.dockerignore Syntax Examples

Lines starting with # are comments and are ignored. Patterns such as /tmp, *.md, and tem? match files or directories, while a leading ! excludes a previously ignored pattern (e.g., !README.md).

Example .dockerignore content:

*.md

!README.md

These rules ignore all markdown files except README.md.

Cache Determination Rules

Docker compares each instruction and its context (including checksums for ADD and COPY) with previous builds; if they match, the cache is used. Therefore, place stable commands like package installation early, and mutable commands like source compilation later.

Example Dockerfile Ordering

FROM centos:7

# Set environment variables early

ENV PATH /usr/local/bin:$PATH

# Install required packages early

RUN yum install -y make

# Place frequently changing application steps at the end

...

Following these principles increases the likelihood of cache hits, significantly reducing image build time.

Timezone Configuration

RUN ln -sf /usr/share/zoneinfo/Asia/Shanghai /etc/localtime

RUN echo "Asia/Shanghai" >> /etc/timezone

These commands set the container’s timezone to China Standard Time.

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