Replace bash script with Cake.Docker task

[HebaruSan]

Motivation

#2838 is setting up build triggers to rebuild an inflator container for #2789. This pull request targets that PR's branch with a suggestion for changing it.

Currently it:

• Relies on a bash script
• Uses the _build/repack/Release folder as its working folder
• Hard-codes paths that are defined as variables in other places, meaning extra maintenance if those ever change

Changes

Now the bash script is replaced with a new task in the build.cake file, which:

• Uses the standard variables to get the path to netkan.exe and _build
• Works out of _build/docker/inflator
• Might work on systems without bash, depending on how portable Cake.Docker is
• Otherwise does the same things the bash script did

The Travis config is updated to rebuild the container with ./build docker-inflator.

This way the new docker logic will fit into the existing build system.

[techman83]

This is great! Thanks for bringing your 🍰 expertise!