Adding dockerfile for testing container and modifying gitlab-ci

Dockerfile-testing

+FROM docker.io/node:8-stretch
+
+ARG DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update \
+    && apt-get install -y chromium
+
+ENV CHROME_BIN=chromium
+
+WORKDIR /usr/src/app
+CMD ["npm", "test:ci"]
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