httpd in docker cannot start - apache

Question

I'm trying to install HTTPD in docker, I wrote a dockerfile like this:
FROM centos
VOLUME /var/log/httpd
VOLUME /etc/httpd
VOLUME /var/www/html
# Update Yum Repostory
RUN yum clean all && \
yum makecache fast && \
yum -y update && \
yum -y install httpd
RUN yum clean all
EXPOSE 80
CMD /usr/sbin/httpd -D BACKGROUND && tail -f /var/log/httpd/access_log
it works if I run the image without host volumes, but failed if I use parameter:
--volume /data/httpd/var/www/html:/var/www/html --volume /data/httpd/var/log:/var/log --volume /data/httpd/etc:/etc/httpd
the error message is:
httpd: Could not open configuration file /etc/httpd/conf/httpd.conf: No such file or directory
I checked the mount point which is empty:
# ll /data/httpd/etc/
total 0
But if I don't use "volume" by default docker copys over files to a temp folder:
# ll /var/lib/docker/volumes/04f083887e503c6138a65b300a1b40602d227bb2bbb58c69b700f6ac753d1c34/_data
total 4
drwxr-xr-x. 2 root root 35 Nov 3 03:16 conf
drwxr-xr-x. 2 root root 78 Nov 3 03:16 conf.d
drwxr-xr-x. 2 root root 4096 Nov 3 03:16 conf.modules.d
lrwxrwxrwx. 1 root root 19 Nov 3 03:16 logs -> ../../var/log/httpd
lrwxrwxrwx. 1 root root 29 Nov 3 03:16 modules -> ../../usr/lib64/httpd/modules
lrwxrwxrwx. 1 root root 10 Nov 3 03:16 run -> /run/httpd
So I'm confused, why docker refused to copy them to the named location? and how to fix this problem?

Answer 1

This is a documented behavior indeed:
Volumes are initialized when a container is created. If the container’s
base image contains data at the specified mount point, that existing data
is copied into the new volume upon volume initialization. (Note that this
does not apply when mounting a host directory.)
i.e. when you mount the /etc/httpd volume --volume /data/httpd/etc:/etc/httpd, no data will be copied.
You can also see https://github.com/docker/docker/pull/9092 for a more detailed discussion on why it works this way (in case you are interested).
A usual workaround for this is to copy your initial data, to the volume folder (from within the container), inside your ENTRYPOINT or CMD script,
in case it is empty.
Note that your initial dataset must be kept outside the volume folder (e.g. as .tar file in /opt), for this to work, as the volume folder will be shadowed by the host folder mounted over it.
Given below is a sample Dockerfile and Script, which demonstrate the behavior:
Sample Dockerfile
FROM debian:stable
RUN mkdir -p /opt/test/; touch /opt/test/initial-data-file
VOLUME /opt/test
Sample script (try various volume mappings)
#Build image
>docker build -t volumetest .
Sending build context to Docker daemon 2.56 kB
Step 0 : FROM debian:stable
---> f7504c16316c
Step 1 : RUN mkdir -p /opt/test/; touch /opt/test/initial-data-file
---> Using cache
---> 1ea0475e1a18
Step 2 : VOLUME /opt/test
---> Using cache
---> d8d32d849b82
Successfully built d8d32d849b82
#Implicit Volume mapping (as defined in Dockerfile)
>docker run --rm=true volumetest ls -l /opt/test
total 0
-rw-r--r-- 1 root root 0 Nov 4 18:26 initial-data-file
#Explicit Volume mapping
> docker run --rm=true --volume /opt/test volumetest ls -l /opt/test/
total 0
-rw-r--r-- 1 root root 0 Nov 4 18:26 initial-data-file
#Explicitly Mounted Volume
>mkdir test
>docker run --rm=true --volume "$(pwd)/test/:/opt/test" volumetest ls -l /opt/test
total 0
And here is a simple entrypoint script, illustrating a possible workaround:
#!/bin/bash
VOLUME=/opt/test
DATA=/opt/data-volume.tar.gz
if [[ -n $(find "$VOLUME" -maxdepth 0 -empty) ]]
then
echo Preseeding VOLUME $VOLUME with data from $DATA...
tar -C "$VOLUME" -xvf "$DATA"
fi
"$#"
add the following to the Dockerfile
COPY data-volume.tar.gz entrypoint /opt/
ENTRYPOINT ["/opt/entrypoint"]
First run:
>docker run --rm=true --volume "$(pwd)/test/:/opt/test" volumetest ls -l /opt/test
Preseeding VOLUME /opt/test with data from /opt/data-volume.tar.gz...
preseeded-data
total 0
-rw-r--r-- 1 1001 users 0 Nov 4 18:43 preseeded-data
Subsequent runs:
>docker run --rm=true --volume "$(pwd)/test/:/opt/test" volumetest ls -l /opt/test
ls -l /opt/test
total 0
-rw-r--r-- 1 1001 users 0 Nov 4 18:43 preseeded-data
Note, that the volume folder will only be populated with data,
if it was completely empty before.