镜像

官方解释：

An image is a read-only template with instructions for creating a Docker container. Often, an image is based on another image, with some additional customization. For example, you may build an image which is based on the image, but installs the Apache web server and your application, as well as the configuration details needed to make your application run.ubuntu
You might create your own images or you might only use those created by others and published in a registry. To build your own image, you create a Dockerfile with a simple syntax for defining the steps needed to create the image and run it. Each instruction in a Dockerfile creates a layer in the image. When you change the Dockerfile and rebuild the image, only those layers which have changed are rebuilt. This is part of what makes images so lightweight, small, and fast, when compared to other virtualization technologies.

我的理解

镜像就像卷心菜，一层包裹一层。你可以在任何镜像上面添加自己的数据，然后build成一个新的镜像。那么你添加的数据就是在原来镜像的基础上面有添加了一层。我举个例子：在安装tomcat时，使用docker pull tomcat你可以看到拉取了多个信息，我们去官网可以看一下tomcat的大小，然后看一下tomcat的镜像大小，会发现镜像远远大于tomcat实际的大小。这是位啥呢？这就是因为tomcat是基于其他镜像而来。比如最基础肯定需要一个操作系统，所以第一层是操作系统，然后tomcat是基于Java的，所以需要jdk运行环境，然后再操作系统的基础上添加一层jdk，然后再是tomcat。这就像卷心菜一样，操作系统被jdk包裹，tomcat包裹jdk，然后你也可以再tomcat上面继续包裹其他数据，然后生成自己的镜像。

dockerfile

我们去官网看看tomcat的dockerfile，链接如下：tomcat/Dockerfile at d7b9f8bc125be41493b68a1618e7137dbe5c04c7 · docker-library/tomcat · GitHub

#
# NOTE: THIS DOCKERFILE IS GENERATED VIA "apply-templates.sh"
#
# PLEASE DO NOT EDIT IT DIRECTLY.
#

FROM eclipse-temurin:17-jre-focal

ENV CATALINA_HOME /usr/local/tomcat
ENV PATH $CATALINA_HOME/bin:$PATH
RUN mkdir -p "$CATALINA_HOME"
WORKDIR $CATALINA_HOME

# let "Tomcat Native" live somewhere isolated
ENV TOMCAT_NATIVE_LIBDIR $CATALINA_HOME/native-jni-lib
ENV LD_LIBRARY_PATH ${LD_LIBRARY_PATH:+$LD_LIBRARY_PATH:}$TOMCAT_NATIVE_LIBDIR

# see https://www.apache.org/dist/tomcat/tomcat-8/KEYS
# see also "versions.sh" (https://github.com/docker-library/tomcat/blob/master/versions.sh)
ENV GPG_KEYS 05AB33110949707C93A279E3D3EFE6B686867BA6 07E48665A34DCAFAE522E5E6266191C37C037D42 47309207D818FFD8DCD3F83F1931D684307A10A5 541FBE7D8F78B25E055DDEE13C370389288584E7 5C3C5F3E314C866292F359A8F3AD5C94A67F707E 765908099ACF92702C7D949BFA0C35EA8AA299F1 79F7026C690BAA50B92CD8B66A3AD3F4F22C4FED 9BA44C2621385CB966EBA586F72C284D731FABEE A27677289986DB50844682F8ACB77FC2E86E29AC A9C5DF4D22E99998D9875A5110C01C5A2F6059E7 DCFD35E0BF8CA7344752DE8B6FB21E8933C60243 F3A04C595DB5B6A5F1ECA43E3B7BBB100D811BBE F7DA48BB64BCB84ECBA7EE6935CD23C10D498E23

ENV TOMCAT_MAJOR 8
ENV TOMCAT_VERSION 8.5.78
ENV TOMCAT_SHA512 b50213e64cc1fd3da2847deda1ca13bee4c26663093c11d53c5ecfe4cdec8856e743b4a1d8488e0c0cbe9bf149e755df40a4140f3b155e2195e3bc6335de3512

COPY --from=tomcat:8.5.78-jdk17-temurin-focal $CATALINA_HOME $CATALINA_HOME
RUN set -eux; \
    apt-get update; \
    xargs -rt apt-get install -y --no-install-recommends < "$TOMCAT_NATIVE_LIBDIR/.dependencies.txt"; \
    rm -rf /var/lib/apt/lists/*

# verify Tomcat Native is working properly
RUN set -eux; \
    nativeLines="$(catalina.sh configtest 2>&1)"; \
    nativeLines="$(echo "$nativeLines" | grep 'Apache Tomcat Native')"; \
    nativeLines="$(echo "$nativeLines" | sort -u)"; \
    if ! echo "$nativeLines" | grep -E 'INFO: Loaded( APR based)? Apache Tomcat Native library' >&2; then \
        echo >&2 "$nativeLines"; \
        exit 1; \
    fi

EXPOSE 8080
CMD ["catalina.sh", "run"]

dockerfile指令含义