Add a new entrypoint to a docker image - docker

Question

Recently, we decided to move one of our services to docker container. The service is product of another company and they have provided us the docker image. However, we need to do some extra configuration steps in the container entrypoint.
The first thing I tried, was to create a DockerFile from the base image and then add commands to do the extra steps, like this:
From baseimage:tag
RUN chmod a+w /path/to/entrypoint_creates_this_file
But, it failed, because these extra steps must be run after running the base container entrypoint.
Is there any way to extend entrypoint of a base image? if not, what is the correct way to do this?
Thanks

Answer 1

I finally ended up calling the original entrypoint bash script in my new entrypoint bash script, before doing other extra configuration steps.

Answer 2

You do not need to even create a new Dockerfile. To modify the entrypoint you can just run the image using the command such as below:
docker run --entrypoint new-entry-point-cmd baseimage:tag <optional-args-to-entrypoint>

Answer 3

create your custom entry-point file
-> add this to image
-> specify this as your entrypoint file
FROM image:base
COPY /path/to/my-entry-point.sh /my-entry-point.sh
// do sth here
ENTRYPOINT ["/my-entry-point.sh"]

Answer 4

Let me take an example with certbot. Using the excellent answer from Anoop, we can get an interactive shell (-ti) into a temporary container (--rm) with this image like so:
$ docker run --rm -ti --entrypoint /bin/sh certbot/certbot:latest
But what if we want to run a command after the original entry point, like the OP requested? We could run a shell and join the commands as in the following example:
$ docker run --rm --entrypoint /bin/sh certbot/certbot:latest \
-c "certbot --version && touch i-can-do-nice-things-here && ls -lah"
certbot 1.30.0
total 28K
drwxr-xr-x 1 root root 4.0K Oct 5 15:10 .
drwxr-xr-x 1 root root 4.0K Sep 7 18:10 ..
-rw-r--r-- 1 root root 0 Oct 5 15:10 i-can-do-nice-things-here
drwxr-xr-x 1 root root 4.0K Sep 7 18:10 src
drwxr-xr-x 1 root root 4.0K Sep 7 18:10 tools
Background
If I run it with the original entrypoint I will get this:
$ docker run --rm certbot/certbot:latest
Saving debug log to /var/log/letsencrypt/letsencrypt.log Certbot
doesn't know how to automatically configure the web server on this
system. However, it can still get a certificate for you. Please run
"certbot certonly" to do so. You'll need to manually configure your
web server to use the resulting certificate.
Or:
$ docker run --rm certbot/certbot:latest --version
certbot 1.30.0
I can see the entrypoint with docker inspect:
$ docker inspect certbot/certbot:latest | grep -i entry -C 2
},
"WorkingDir": "/opt/certbot",
"Entrypoint": null,
"OnBuild": null,
"Labels": null
--
},
"WorkingDir": "/opt/certbot",
"Entrypoint": [
"certbot"
],
If /bin/sh doesn't work in your container, try /bin/bash.