The caching feature of docker image and dockerfile

One 、Docker The caching characteristics of the image

1、 Build a mirror image

Docker It will cache the image layer of the existing image , When building a new image , If a mirror layer already exists , Just use it , No need to recreate .

for example ：

In front of Dockerfile Add a little new content to , Copy a file into the image imageFuGai：

vi Dockerfile

# This my first CentOS Dockerfile

# Version 1.0

# Base images

FROM centos

 

#MAINTAINER

MAINTAINER zola

 

#ENV

#ADD

 

#RUN

RUN yum install -y wget

 

COPY  imageFuGai

 

#WORKDIR

#EXPOSE

#CMD

 

[[email protected] ~]# pwd

/root

[[email protected] ~]# ll

total 8

-rw-------. 1 root root 1616 Apr 29 17:42 anaconda-ks.cfg

-rw-r--r--. 1 root root  189 May 10 20:09 Dockerfile

-rw-r--r--. 1 root root    0 May 10 20:02 imageFuGai

[[email protected] ~]# docker build -t centos-with-wget-02  .

Sending build context to Docker daemon  17.41kB

Step 1/4 : FROM centos

 ---> 470671670cac

Step 2/4 : MAINTAINER zola

 ---> Using cache

 ---> fe6ec852b0c9

Step 3/4 : RUN yum install -y wget

 ---> Using cache

 ---> 1cca79f8389d

Step 4/4 : COPY  imageFuGai  /

 ---> e0ebf5dc3b3c

Successfully built e0ebf5dc3b3c

Successfully tagged centos-with-wget-02:latest

[[email protected] ~]#

explain ：

Step 2 and Step 3： The same has been run before RUN Instructions , This time, use the image layer in the cache directly fe6ec852b0c9 and 1cca79f8389d.

Step 4： perform COPY Instructions .

The process is to start the temporary container , Copy imageFuGai, Commit a new mirror layer e0ebf5dc3b3c, Delete temporary container .

Do not use cache when building images , Can be in docker build Add... To the order --no-cache Parameters

docker build -t --no-cache  centos-with-wget-02  .

Dockerfile Each instruction in creates a mirror layer , The upper layer depends on the lower layer . whenever , As long as one layer changes , The cache of all layers above it will be invalidated .

If the change Dockerfile The order in which instructions are executed , Or modify or add instructions , Will invalidate the cache .

2、 Download mirroring

Docker Cache will also be used when downloading images . For example, we download httpd Mirror image

docker  history  debian

 

Two 、 debugging Dockerfile

1、 adopt Dockerfile The process of building a mirror

from base Mirror runs a container

Execute an order , Modify the container

Execution is similar to docker commit The operation of , Create a new mirror layer

Docker Then run a new container based on the image just submitted

repeat 2-4 Step , until Dockerfile All instructions in are executed

2、Dockerfile For some reason, the execution of an instruction fails , We will also be able to get the image constructed by the successful execution of the previous instruction , This is for debugging Dockerfile Very helpful

If Dockerfile In the third step RUN Instructions 1 When the failure . We can use the image created in step 2 111133223 debug , The way is through docker run -it Start a container for the image

docker run -it   111133223

By hand RUN Instructions 1 It's easy to locate the cause of failure

3、 ... and 、Dockerfile Commonly used instructions

Reference link ：

https://www.runoob.com/docker/docker-dockerfile.html

FROM

Appoint base Mirror image .

MAINTAINER

Set the author of the mirror , It could be any string .

 

COPY

The file from build context Copy to mirror .

COPY Support two forms ：

COPY src dest

COPY ["src", "dest"]

Be careful ：src Only... Can be specified build context Files or directories in .

 

ADD

And COPY similar , from build context Copy the file to the image . The difference is , If src It's an archive （tar, zip, tgz, xz etc. ）, The file will be automatically unzipped to dest.

 

ENV

Set the environment variable , Environment variables can be used by subsequent instructions . for example ：

...

ENV MY_VERSION 1.3

RUN yum -y install -y mypackage=$MY_VERSION

...

 

EXPOSE

Specifies that the process in the container will listen on a port ,Docker You can expose the port . We will discuss in detail in the container network section .

 

VOLUME

Declare a file or directory as volume. We will discuss in detail in the container storage section .

 

WORKDIR

For the back of the RUN, CMD, ENTRYPOINT, ADD or COPY Directive sets the current working directory in the image .

 

RUN

Runs the specified command in the container .

 

CMD

Run the specified command when the container starts .

Dockerfile There can be more than one CMD Instructions , But only the last one works .CMD Can be docker run After the parameter replacement .

 

ENTRYPOINT

Sets the command to run when the container starts .

Dockerfile There can be more than one ENTRYPOINT Instructions , But only the last one works .CMD or docker run Subsequent parameters are passed as parameters to the ENTRYPOINT.

notes ：Dockerfile Support with “#” Opening comment

Be careful ：Dockerfile Every time an instruction is executed, it will be in docker Build a new floor on it . So there are too many meaningless layers , It will cause the mirror image to expand too much . for example ：

 

FROM centos

RUN yum install wget

RUN wget -O redis.tar.gz "http://download.redis.io/releases/redis-5.0.3.tar.gz"

RUN tar -xvf redis.tar.gz

The above execution will create 3 Layer mirror . It can be simplified to the following format ：

FROM centos

RUN yum install wget \

    && wget -O redis.tar.gz "http://download.redis.io/releases/redis-5.0.3.tar.gz" \

    && tar -xvf redis.tar.gz

With && Symbolic link command , After this , Only create 1 Layer mirror

RUN、CMD、ENTRYPOINT It's important and confusing

Four 、RUN、CMD and ENTRYPOINT

RUN Execute the command and create a new mirror layer ,RUN Often used to install software packages

CMD Set the default command and its parameters after the container is started , but CMD It can be docker run Replace with the command line argument

ENTRYPOINT Configure the commands to run when the container starts

1、Shell and Exec Format

There are two ways to specify RUN、CMD and ENTRYPOINT Commands to run ：Shell Format and Exec Format , There are subtle differences in the use of the two .

Shell Format

<instruction> <command>

Exec Format

<instruction> ["executable", "param1", "param2", ...]

CMD and ENTRYPOINT Recommended Exec Format , Because the instructions are more readable , Easier to understand .RUN Two formats shell Format and Exec Format is OK

2、RUN There are two formats

Shell Format ：RUN

Exec Format ：RUN ["executable", "param1", "param2"]

Use RUN Install multiple packages ：

RUN apt-get update && apt-get install -y \  

 bzr \

 cvs \

 git \

 mercurial \

 subversion

Be careful ：apt-get update and apt-get install Put in a RUN To execute in an instruction , This ensures that the latest package is installed every time . If apt-get install In a separate RUN In the implementation of , Will use apt-get update Create a mirror layer , This layer may have been cached a long time ago .

3、 Best practices

Use RUN Instructions to install applications and software packages , Build a mirror image .

If Docker The purpose of the image is to run applications or services , Like running a MySQL, Should be used first Exec Format ENTRYPOINT Instructions .CMD for ENTRYPOINT Provide additional default parameters , Simultaneously available docker run The command line replaces the default parameters .

If you want to set the default startup command for the container , You can use CMD Instructions . The user can be in docker run Replace this default command on the command line .

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