构建centos-wget 镜像前本地没有缓存centos镜像，在构建的第一步需要dokcer pull centos镜像
root@docker-lab:~/docker#
cat Dockerfile
FROM centos
RUN yum install -y wget
root@docker-lab:~/docker#
docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
root@docker-lab:~/111#
docker build -t centos-wget .
Sending build context to Docker daemon 2.048kB
Step 1/2 : FROM centos
latest: Pulling from library/centos
a02a4930cb5d: Pull complete
Digest: sha256:365fc7f33107869dfcf2b3ba220ce0aa42e16d3f8e8b3c21d72af1ee622f0cf0
Status: Downloaded newer image for centos:latest
---> 1e1148e4cc2c
Step 2/2 : RUN yum install -y wget
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构建centos-wget 镜像前本地有缓存centos镜像，在构建的第一步直接使用本地缓存，无需docker pull
root@docker-lab:~/docker#
cat Dockerfile
FROM centos
RUN yum install -y wget
root@docker-lab:~/docker#
docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
root@docker-lab:~/docker#
docker pull centos
Using default tag: latest
latest: Pulling from library/centos
a02a4930cb5d: Pull complete
Digest: sha256:38777a4106f0072649128ea4236241345a3ed21c55abbbd53bad5342549f6126
Status: Downloaded newer image for centos:latest
root@docker-lab:~/docker# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
centos latest 1e1148e4cc2c 4 weeks ago 202MB
root@docker-lab:~/docker#
docker build -t centos-wget .
Sending build context to Docker daemon 2.048kB
Step 1/2 : FROM centos
---> 1e1148e4cc2c
Step 2/2 : RUN yum install -y wget
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继续构建一个新的镜像，比centos-wget多执行一个uptime，可以看到第一步和第二步直接使用的缓存，并未重新构建
root@docker-lab:~/docker#
cat Dockerfile
FROM centos
RUN yum install -y wget
RUN uptime
root@docker-lab:~/docker#
docker build -t centos-uptime .
Sending build context to Docker daemon 2.048kB
Step 1/3 : FROM centos
---> 1e1148e4cc2c
Step 2/3 : RUN yum install -y wget
---> Using cache
---> 31093d4c7d97
Step 3/3 : RUN uptime
---> Running in 303cb5e38dd3
16:35:42 up 203 days, 7:58, 0 users, load average: 0.00, 0.00, 0.00
Removing intermediate container 303cb5e38dd3
---> cb7f8bec5d08
Successfully built cb7f8bec5d08
Successfully tagged centos-uptime:latest
root@docker-lab:~/docker#
docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
centos-uptime latest cb7f8bec5d08 11 seconds ago 272MB
centos-wget latest 31093d4c7d97 7 minutes ago 272MB
centos latest 1e1148e4cc2c 4 weeks ago 202MB
如果将yum和uptime两个对调，则不会使用之前的 centos-wget 镜像缓存
root@docker-lab:~/docker#
cat Dockerfile
FROM centos
RUN uptime
RUN yum install -y wget
root@docker-lab:~/docker#
docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
centos-uptime latest cb7f8bec5d08 5 minutes ago 272MB
centos-wget latest 31093d4c7d97 12 minutes ago 272MB
centos latest 1e1148e4cc2c 4 weeks ago 202MB
root@docker-lab:~/docker#
docker build -t centos-test .
Sending build context to Docker daemon 2.048kB
Step 1/3 : FROM centos
---> 1e1148e4cc2c
Step 2/3 : RUN uptime
---> Running in 1724b8f64a50
16:41:11 up 203 days, 8:03, 0 users, load average: 0.00, 0.02, 0.00
Removing intermediate container 1724b8f64a50
---> 91a208dedb3d
Step 3/3 : RUN yum install -y wget
---> Running in 85f7f29637d5
Loaded plugins: fastestmirror, ovl
Determining fastest mirrors
* base: mirrors.huaweicloud.com
* extras: mirrors.huaweicloud.com
* updates: mirrors.huaweicloud.com
Resolving Dependencies
--> Running transaction check
---> Package wget.x86_64 0:1.14-18.el7 will be installed
--> Finished Dependency Resolution
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root@docker-lab:~/docker#
docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
centos-test latest fc8b11ebfd31 About a minute ago 272MB
centos-uptime latest cb7f8bec5d08 7 minutes ago 272MB
centos-wget latest 31093d4c7d97 14 minutes ago 272MB
centos latest 1e1148e4cc2c 4 weeks ago 202MB
总结：
1、构建镜像时，FROM 中的镜像如果本地有，就会直接使用本地缓存
2、构建镜像时，如果Dockerfile前N行与之前某镜像构建时Dockerfile的前N行一样，那将直接使用之前镜像中的层，不比从头开始构建。除非在docker build 中添加 --no-cache 参数