Rebuild Docker container on file changes

Question

For running an ASP NET Core application  I generated a dockerfile which build the application and copys the source code in the container  which is fetched by Git using Jenkins  So in my workspace  I do the following in the dockerfile    WORKDIR  app COPY src src   While Jenkins updates the files on my host correctly with Git  Docker doesn t apply this to my image    My basic script for building      bin bash imageName xx my-image containerName my-container  docker build -t  imageName -f Dockerfile     containerRunning   docker inspect --format      State Running      containerName 2 gt   dev null   if     containerRunning      true     then         docker stop  containerName         docker start  containerName else         docker run -d -p 5000 5000 --name  containerName  imageName fi   I tried different things like --rm and --no-cache parameter for docker run and also stopping removing the container before the new one is build  I m not sure what I m doing wrong here  It seems that docker is updating the image correctly  as the call of COPY src src would result in a layer id and no cache call    Step 6   COPY src src  --- gt  382ef210d8fd   What is the recommended way to update a container    My typical scenario would be  The application is running on the server in a Docker container  Now parts of the app are updated  e g  by modifying a file  Now the container should run the new version  Docker seems to recommend building a new image instead of modifying a existing container  so I think the general way of rebuilding like I do is right  but some detail in the implementation has to be improved

User · Answer

You can run build for a specific service by running docker-compose up --build  lt service name gt  where the service name must match how did you call it in your docker-compose file  Example Let s assume that your docker-compose file contains many services   net app - database - let s encrypt    etc  and you want to update only the  net app which named as application in docker-compose file  You can then simply run docker-compose up --build application Extra parameters In case you want to add extra parameters to your command such as -d for running in the background  the parameter must be before the service name  docker-compose up --build -d application

User · Answer

Whenever changes are made in dockerfile or compose or requirements   re-Run it using docker-compose up --build   So that images get rebuild and refreshed

User · Answer

After some research and testing  I found that I had some misunderstandings about the lifetime of Docker containers  Simply restarting a container doesn t make Docker use a new image  when the image was rebuilt in the meantime  Instead  Docker is fetching the image only before creating the container  So the state after running a container is persistent    Why removing is required  Therefore  rebuilding and restarting isn t enough  I thought containers works like a service  Stopping the service  do your changes  restart it and they would apply  That was my biggest mistake   Because containers are permanent  you have to remove them using docker rm  lt ContainerName gt  first  After a container is removed  you can t simply start it by docker start  This has to be done using docker run  which itself uses the latest image for creating a new container-instance    Containers should be as independent as possible  With this knowledge  it s comprehensible why storing data in containers is qualified as bad practice and Docker recommends data volumes mounting host directorys instead  Since a container has to be destroyed to update applications  the stored data inside would be lost too  This cause extra work to shutdown services  backup data and so on   So it s a smart solution to exclude those data completely from the container  We don t have to worry about our data  when its stored safely on the host and the container only holds the application itself    Why -rf may not really help you  The docker run command  has a Clean up switch called -rf  It will stop the behavior of keeping docker containers permanently  Using -rf  Docker will destroy the container after it has been exited  But this switch has two problems     Docker also remove the volumes without a name associated with the container  which may kill your data Using this option  its not possible to run containers in the background using -d switch   While the -rf switch is a good option to save work during development for quick tests  it s less suitable in production  Especially because of the missing option to run a container in the background  which would mostly be required   How to remove a container  We can bypass those limitations by simply removing the container   docker rm --force  lt ContainerName gt    The --force  or -f  switch which use SIGKILL on running containers  Instead  you could also stop the container before    docker stop  lt ContainerName gt  docker rm  lt ContainerName gt    Both are equal  docker stop is also using SIGTERM  But using --force switch will shorten your script  especially when using CI servers  docker stop throws an error if the container is not running  This would cause Jenkins and many other CI servers to consider the build wrongly as failed  To fix this  you have to check first if the container is running as I did in the question  see containerRunning variable     Full script for rebuilding a Docker container  According to this new knowledge  I fixed my script in the following way       bin bash imageName xx my-image containerName my-container  docker build -t  imageName -f Dockerfile     echo Delete old container    docker rm -f  containerName  echo Run new container    docker run -d -p 5000 5000 --name  containerName  imageName   This works perfectly

[git] Rebuild Docker container on file changes

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