Added custom local apps

[dhitchenor]

Adds the ability to bind a second apps directory, so the user can add apps locally Fixes issue #43

docker run -it \
-e SERVER_BRANCH=master \
-e CUSTOM_LOCALAPP=myapps \
--name nextcloud-easy-test \
-p 127.0.0.1:8443:443 \
--volume="nextcloud_easy_test_npm_cache_volume:/var/www/.npm" \
--volume="/path/to/local/directory:/var/www/nextcloud/myapps" \
ghcr.io/szaimen/nextcloud-easy-test:latest

Something to that effect, at least.

HTH