Essential Docker Image Building Tips to Shrink Size and Speed Up Builds

Build Context

When running docker build, the current working directory is sent as the build context; therefore the directory must be clean. Use a dedicated project folder and place only the resources needed for the image.

You can also use a .dockerignore file to exclude unnecessary files from the context.

Base Images

Choose small base images such as alpine or debian:buster-slim. For OpenJDK, use openjdk:xxx-slim which is based on debian:xxx-slim. Note that Alpine uses musl libc instead of glibc, which may cause compatibility issues for large projects that depend on glibc (e.g., OpenJDK, Tomcat, RabbitMQ).

Regional Package Mirrors

Replace default package sources with domestic mirrors to speed up installation. Examples:

echo "http://mirrors.huaweicloud.com/alpine/latest-stable/main/" > /etc/apk/repositories \
 echo "http://mirrors.huaweicloud.com/alpine/latest-stable/community/" >> /etc/apk/repositories \
 apk update

sed -i 's/deb.debian.org/mirrors.huaweicloud.com/g' /etc/apt/sources.list \
 sed -i 's|security.debian.org/debian-security|mirrors.huaweicloud.com/debian-security|g' /etc/apt/sources.list \
 apt update

sed -i 's/archive.ubuntu.com/mirrors.huaweicloud.com/g' /etc/apt/sources.list \
 apt update

Timezone Settings

Most base images default to UTC, which differs from Beijing time by eight hours. Set the container timezone via the environment variable -e TZ=Asia/Shanghai or install tzdata and copy the zoneinfo file.

# Alpine example
apk add --no-cache tzdata \
 cp /usr/share/zoneinfo/Asia/Shanghai /etc/localtime \
 echo "Asia/Shanghai" > /etc/timezone \
 apk del tzdata

# Debian/Ubuntu example
docker run --rm -it -e TZ=Asia/Shanghai debian date

Adding Source Code Efficiently

Prefer git or wget to fetch source code during the build and delete it afterwards, rather than using ADD or COPY which permanently adds the files to a layer.

git clone https://example.com/project.git --depth 1 \
 make && make install \
 rm -rf project

FastDFS Dockerfile Optimization Example

Original Dockerfile (CentOS) results in an image of ~500 MB. After merging all RUN commands, using a virtual build environment, and switching to alpine, the image size drops to ~30 MB.

# Optimized Dockerfile (Alpine)
FROM alpine:3.10
RUN set -x \
    && echo "http://mirrors.huaweicloud.com/alpine/latest-stable/main/" > /etc/apk/repositories \
    && echo "http://mirrors.huaweicloud.com/alpine/latest-stable/community/" >> /etc/apk/repositories \
    && apk update \
    && apk add --no-cache --virtual .build-deps gcc libc-dev make perl-dev openssl-dev pcre-dev zlib-dev git \
    && mkdir -p /usr/local/src \
    && cd /usr/local/src \
    && git clone https://github.com/happyfish100/libfastcommon.git --depth 1 \
    && git clone https://github.com/happyfish100/fastdfs.git --depth 1 \
    && git clone https://github.com/happyfish100/fastdfs-nginx-module.git --depth 1 \
    && wget http://nginx.org/download/nginx-1.15.4.tar.gz \
    && tar -xf nginx-1.15.4.tar.gz \
    && cd libfastcommon && ./make.sh && ./make.sh install \
    && cd ../fastdfs && ./make.sh && ./make.sh install \
    && cd ../nginx-1.15.4 && ./configure --add-module=/usr/local/src/fastdfs-nginx-module/src/ && make && make install \
    && apk del .build-deps \
    && apk add --no-cache pcre-dev bash \
    && mv /usr/local/src/fastdfs/docker/dockerfile_network/fastdfs.sh /home \
    && mv /usr/local/src/fastdfs/docker/dockerfile_network/conf/* /etc/fdfs \
    && chmod +x /home/fastdfs.sh \
    && rm -rf /usr/local/src*
VOLUME /home/dfs
EXPOSE 22122 23000 8888 8080
CMD ["/home/fastdfs.sh"]

# Image size comparison
fastdfs   alpine   29.3MB
fastdfs   debian   103MB
fastdfs   centos   483MB

Virtual Build Environment

Install temporary build dependencies with apk add --no-cache --virtual .build-deps … and remove them after the build using apk del .build-deps. Keep runtime dependencies separate.

Minimizing Layers

Only RUN, COPY, and ADD create image layers. Combine multiple commands in a single RUN using && or ;. Reduce the number of ADD instructions by packaging files into a tarball and extracting them in one layer.

# Example of reducing many ADDs to one
COPY src.tar.gz /usr/local/src.tar.gz
RUN tar -xvf /usr/local/src.tar.gz -C /usr/local && \
    mv /usr/local/src/conf/* /etc/fdfs && \
    rm -rf /usr/local/src/* /var/cache/apk/*