Added WORKDIR variable

[mindthecap]

In order to circumvent the problem that from version 15 of nodejs, npm install is triggered from root directory, a WORKDIR variable has to be used as explained in https://stackoverflow.com/questions/57534295/npm-err-tracker-idealtree-already-exists-while-creating-the-docker-image-for. This fixes the issue "npm ERR! Tracker "idealTree" already exists" that comes during build.

[CLAassistant]

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