Docker :9

DevOps Classroom Series – 28/Mar/2021

USER, HEALTHCHECK and EXPOSE instructions

• Docker will use the root as the default user in the docker containers
• USER instruction can change this behavior and specify a non -root user ad default user

USER <user>
USER <user>:group

• Lets create a Dockerfile based on Apache Server Refer Here for the Dockerfile
• Now build the docker image

docker image build -t userdemo:1.0 .

• Create a container and keep it running in the background  
• EXPOSE instruction is used to inform Docker that a container is listenting on the specified port at run time

EXPOSE <port>
EXPOSE <port>/<protocol>

• Ports exposed Expose instruction will only be accesible within docker container
• To access the ports from the host we can use -p <host-port>:<container-port> or -P 
• To verify if the application is running or not we can create HEALTHCHECK insturction

HEALTHCHECK --internal=1m --timeout=2s --retries=3 CMD curl -f http://localhost/ || exit 1

• Lets remove all the containers  
• Lets remove all the images 
• All the containers are deleted as well as the images
• Lets understand how image layers impact docker container
• Refer Here for the changeset containing the dockerfile

Docker Storage

• The image layers are organized as shown below 
• Lets pull httpd and create 3 containers  
• Now lets run the docker container ls -s 
• SIZE is the actual size of the r/w layer and the virtual size the combined size of Image layer + R/W Layer 
• Docker use Copy-on-write strategy (COW) to make changes in the existing files in the image layers
• To make this layers work docker uses storage drivers. Docker supports the following storage drivers
  • overlay2
  • aufs
  • devicemapper
  • brtfs storage driver
  • zfs storage drivers
• Refer Here for the official documentation.
• When the docker container is deleted the R/W layer is deleted and if the R/W layer is deleted we loose the data generated
• Before trying to resolve this lets try to understand docker container states   
• In all of the above states apart from Removed/Dead docker container data is still available on the docker host
• But once we remove the container the data generated in the R/W layer will be deleted 

Docker Volumes

• We can use docker volumes to save the persisted data without relying on containers lifecycle
• If we create a docker volume and attach it to the container, even if the container is deleted the volume will still be available
• First option of creating a volume can be done in the Dockerfile, we can use the VOLUME instruction. Refer Here for the docker file
• Now lets build the image

docker image build -t voldemo:1.0 .

• Lets execute the following command

docker volume ls

• now lets create a container from this image 
• Now lets create some dummy files in the /var/www/html folder 
• Now lets inspect volume  
• Now lets remove the container 
• Now even after we remove the container the volume is still present on the docker host and the data in the /var/www/html folder is retained.
• But this might not be the case as the Dockerfile will not contain volume instruction in all of the cases, so we need to understand how to use Docker volumes by attaching volumes to the containers
• To attach the volumes to the docker container we need to understand what are different mount types supported by the docker volumes
  • bind mount
  • volume mount
  • tmpfs mount 
• In docker mounts can be done by two kinds of commands
  • -v
  • –mount
• Bind mount
  • for official documentation Refer Here
• Lets create the container with bind mount

docker container run -d --name bindmountapache --mount "type=bind,source=/root/html,target=/var/www/html" -P httpd

docker container run -d --name bindmountapache2 -v "/root/html:/var/www/html" -P httpd

• To make the mounts readonly

docker container run -d --name bindmountapache3 --mount "type=bind,source=/root/html,target=/var/www/html,readonly" -P httpd

docker container run -d --name bindmountapache4 -v "/root/html:/var/www/html:ro" -P httpd