Docker containers retaining data

I'm learning Docker and I've been crafting a Dockerfile for an Ubuntu container.

My problem is I keep getting persistent information between different containers. I have exited, removed the container and then removed its image. After I made changes to my Dockerfile, I executed docker build -t playbuntu . Executing the following Dockerfile:

FROM ubuntu:latest
        
    ## for apt to be noninteractive
    ENV DEBIAN_FRONTEND noninteractive
    ENV DEBCONF_NONINTERACTIVE_SEEN true
    
    ## preesed tzdata, update package index, upgrade packages and install needed software
    RUN echo "tzdata tzdata/Areas select Europe" > /tmp/preseed.txt; \
        echo "tzdata tzdata/Zones/Europe select London" >> /tmp/preseed.txt; \
        debconf-set-selections /tmp/preseed.txt && \
        apt-get update && \
        apt-get install -y tzdata
    
    
    RUN apt-get update -y && apt-get upgrade -y && apt-get install tree nano vim -y && apt-get install less -y && apt-get install lamp-server^ -y
    
    RUN echo "ServerName localhost" >> /etc/apache2/apache2.conf
    
    EXPOSE 80
    
    WORKDIR /var/www
    
    COPY ./index.php /var/www
    COPY ./000-default.conf /etc/apache2/sites-available
    
    CMD [ "apache2ctl", "start", "&&", "apache2ctl", "restart" ]

Once I execute winpty docker run -it -p 80:80 playbuntu bash, my problem is, instead of my index.php file outputting the following:

<?php

    print "<center><h3>Sisko's LAMP activated!</h3><center>";
    
    phpinfo();

I get the following debug code I experimented with hours ago:

<?php

    print "...responding";
    
    phpinfo();

Is there a caching system Docker might be using? I pruned all Docker volumes just in case that is how Docker might be caching. All except 2 volumes which are being used by other containers unrelated to my project.

I suspect that, after you changed index.php on your host machine you did not rerun the docker build ... command. You will need to recreate the container image any time any of the image's content changes.

Please confirm whether running the docker build ... command again solves the issue.

Background

Docker images comprise one of more layers.

Image (layers) and Volumes are distinct.

Since your docker run ... command contains no e.g. --volume= bind mounts (or equivalent), I suspect Docker Volumes are not relevant to this question.

There is a possibility that rebuilding images does not replace image layers and thus caching does occur. However, in your case, I think this is not the issue. See the Docker documentation (link) for an overview of the Dockerfile commands that add layers.

Because of the way that layers work, if the preceding layers in your Dockerfile were unchanged and the index.php were unchanged, then Docker would not rebuild these layers. However, because your Dockerfile includes a layer that apt-get update && apt-get install ...., the layer will be invalidated, recreated and so will subsequent layers.

If you change index.php on the host and rebuild the image, this layer will always be rebuilt.

I built your Dockerfile twice. Here's the beginning of the second (!) build command. NB the Using cache commands for the unchanged layers preceding the RUN apt-get update... layer which is rebuilt.

docker build --rm -f "Dockerfile" -t 59582820:0939 "."
Sending build context to Docker daemon  3.584kB
Step 1/10 : FROM ubuntu:latest
 ---> 549b9b86cb8d
Step 2/10 : ENV DEBIAN_FRONTEND noninteractive
 ---> Using cache
 ---> 1529d0e293f3
Step 3/10 : ENV DEBCONF_NONINTERACTIVE_SEEN true
 ---> Using cache
 ---> 1ba10410d06a
Step 4/10 : RUN echo "tzdata tzdata/Areas select Europe" > /tmp/preseed.txt;     echo "tzdata tzdata/Zones/Europe select London" >> /tmp/preseed.txt;     debconf-set-selections /tmp/preseed.txt &&     apt-get update &&     apt-get install -y tzdata
 ---> Using cache
 ---> afb861da52e4
Step 5/10 : RUN apt-get update -y && apt-get upgrade -y && apt-get install tree nano vim -y && apt-get install less -y && apt-get install lamp-server^ -y
 ---> Running in 6f05bbb8e80a

The evidence suggests to me that you didn't rebuild after changing the index.php.

You can use volumes to persist some folders like configs or properties etc. Docker Storage