Add env options to build script

[rgstephens]

Add env options to the build script to make it easier to use the demo with your own data.

Var	Use
DATA_IMAGE_NAME	Elastic docker image, default is deepset/elasticsearch-countries-and-capitals
DATASET_DIR	Data/Corpus directory, default is dataset
HAYSTACK_IMAGE_NAME	Haystack build image, default is deepset/haystack:cpu-main
NETWORK	Docker network name, default is explore_the_world

[julian-risch]

fixed by PR #3