JULIA_DEPOT_PATH setting after precompilation reduces container startup performance

In the Dockerfile created by this plugin, the following setup reduces performance: Initially, packages are precompiled with
```
RUN julia -e "using Pkg; Pkg.activate(\\".\\"); Pkg.instantiate(); Pkg.precompile(); "
```

_After_ this statement, the Julia depot path is changed with
```
ENV JULIA_DEPOT_PATH "/home/$user/.julia"
```

When starting a container, Julia will look for cached compiled package images in the new depot path and will not find any (as they reside in the default location used for the first statement, before setting the env variable). This will trigger a unnecessary recompilation.

Inspect the generated Dockerfile and the order of the Julia precompilation command and JULIA_DEPOT_PATH setting. Ensure the depot path used during precompilation matches the path used when the container starts, then verify that startup does not trigger unnecessary recompilation.

Written by the indexing model from the issue text.