# How to make the smallest OCaml Docker container image
OCaml used to use a build tool called `jbuilder`.
However, since 2018, it has been replaced by a project called `dune`.
This page shows how to build an application with `dune` and create a minimal Docker container image.
See: [dune migration](https://dune.readthedocs.io/en/latest/migration.html?highlight=Jbuilder#migration)
## Install opam
`opam` is a package management tool for OCaml.
You should install it beforehand, because you use it to install OCaml libraries (modules) and tools such as `dune`.
Execute the following command to install `opam`.
Note that the environment to install is `Ubuntu 20.04`.
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