how should I persistently save Julia packages in a Docker container

You can manage the package and their versions via a Julia Project.toml file.This file can keep both the list of your dependencies.

Here is a sample Julia session:

julia> using Pkgjulia> pkg"generate MyProject" Generating  project MyProject:    MyProject\Project.toml    MyProject\src/MyProject.jljulia> cd("MyProject")julia> pkg"activate ." Activating environment at `C:\Users\pszufe\myp\MyProject\Project.toml`julia> pkg"add DataFrames"

Now the last step is to provide package version information to your Project.toml file. We start by checking the version number that "works good":

julia> pkg"st DataFrames"Project MyProject v0.1.0Status `C:\Users\pszufe\myp\MyProject\Project.toml`  [a93c6f00] DataFrames v0.21.7

Now you want to edit Project.toml file [compat] to fix that version number to always be v0.21.7:

name = "MyProject"uuid = "5fe874ab-e862-465c-89f9-b6882972cba7"authors = ["pszufe <pszufe@******.com>"]version = "0.1.0"[deps]DataFrames = "a93c6f00-e57d-5684-b7b6-d8193f3e46c0"[compat]DataFrames = "= 0.21.7"

Note that in the last line the equality operator is twice to fix the exact version number see also https://julialang.github.io/Pkg.jl/v1/compatibility/.

Now in order to reuse that structure (e.g. different docker, moving between systems etc.) all you do is

cd("MyProject")using Pkgpkg"activate ."pkg"instantiate"

Additional note

Also have a look at the JULIA_DEPOT_PATH variable (https://docs.julialang.org/en/v1/manual/environment-variables/).When moving installations between dockers here and there it might be also sometimes convenient to have control where all your packages are actually installed. For an example you might want to copy JULIA_DEPOT_PATH folder between 2 dockers having the same Julia installations to avoid the time spent in installing packages or you could be building the Docker image having no internet connection etc.

You can persist the state of downloaded & precompiled packages by mounting a dedicated volume into /home/your_user/.julia inside the container:

$ docker run --mount source=dot-julia,target=/home/your_user/.julia [OTHER_OPTIONS]

Depending on how (and by which user) julia is run inside the container, you might have to adjust the target path above to point to the first entry in Julia's DEPOT_PATH.

You can control this path by setting it yourself via the JULIA_DEPOT_PATH environment variable. Alternatively, you can check whether it is in a nonstandard location by running the following command in a Julia REPL in the container:

julia> println(first(DEPOT_PATH))/home/francois/.julia