Modifying a docker image that was created from an existing one

Just create a new Dockerfile in an empty Directory. Start the Dockerfile with

FROM repo/image 

where 'repo/image' is the id of the image your are currently using.

and add your customizations below.

This way you build a new image that is based on another image.

In particular, to change the command that runs at startup, put in a CMD and/or ENTRYPOINT line.

To answer your specific q: "the container runs a bash script when it starts, i want to change this". Let's assume you want to run /script.sh (part of the image) instead of the default, you can instantiate a container using:

docker run --entrypoint /script.sh repo/image

If script.sh isn't part of the image and/or you prefer not having to specify it explicitly each time with --entrypoint as above, you can prepare an image that contains and runs your own script.sh:

1. Create an empty directory and copy or create script.sh in it

2. Create Dockerfile with following content:

   FROM repo/image
   ADD script.sh /
   ENTRYPOINT /script.sh
   

3. docker build -t="myimage" .

4. docker run myimage

Notes:

• When running the container (step 4), it's no longer necessary to specify --entrypoint since we have it defaulted in the Dockerfile.
• It's really that simple; no need to sign up to docker hub or any such thing (although it's of course recommended in time ;-)