Installing in a docker

I'm trying to automate the installation of `fundle` and `pure` theme in a container but my prompt isn't activated.

The base Docker image I use is [ohmyfish/fish](https://hub.docker.com/r/ohmyfish/fish/~/dockerfile/) and the command executed are:

```
curl -sfL https://git.io/fundle-install | fish
fundle plugin rafaelrinaldi/pure
fundle install
fish_prompt
```

At the end of that script my prompt is the same as at the beginning. I was hoping it would load `pure` theme.

Reproduce the container setup using the ohmyfish/fish Docker image and the listed curl, fundle, and fish_prompt commands. Trace how installation and prompt activation are handled in the shell environment, then verify that the pure theme is loaded and the container prompt changes as expected.

Written by the indexing model from the issue text.