For example, apt install leaves you with a lot of cache junk in /var (same for the other package managers).

For each RUNcall, make sure there are no temporary/unneccessary files left afterwards.

Switch the base image to a smaller one.

If you just do it naively, you will end up with a huge image (gigabytes), and create a new huge image everytime you rebuild, wich can pose serious problems.

The last step is to (optionally) create a ENTRYPOINT, i.e., a command that is run when the image is started up, as well as EXPOSE routing information, run as non-root USER or set ENVironment variables.

If you follow the usual approach, you would prepare any binaries and other files you need in the host-side directory alongside your Dockerfile, and then ADD them in.

Finally, you have to install your own software somehow.

(or whatever package manager your base image uses) to add any standard packages you like. Hence, your next step is to use RUN apt install.

Those base images usually are pretty small.

At first, you can pick any Debian, Ubuntu or whatever distribution you prefer and are used to. Depending on your needs and security requirements, you can pick your favourite distribution from Dockerhub.

That is, you chain a lot of commands, which are executed during the build process, and whatever ends up in the file system at the end gets packed up into an image. If yes, what are the big lines to follow?Ī Dockerfile is just a funky version of a plain old shell script which you could ordinarily use to provision your software with all its dependencies. I.e., all OS libraries (libc, everything else.) are in there anyways. While it is true that a Docker image is not a standalone VM, it still contains everything an equivalent VM would contain - excluding the kernel. While Docker is often used for microservice-oriented images (i.e., one single service per image), nothing keeps you from converting a monolithic old application to Docker by building a large image, and running arbitrarily many processes inside.

Ubuntu server vdi image