Add .gitlab-ci.yml with single stage for build jobs

For now, use Dockerhub as registry.

.gitlab-ci.yml

+image: docker:stable
+
+services:
+  - docker:dind
+
+variables:
+    BACKEND_DIR: api
+    FRONTEND_DIR: client
+    NODE_IMAGE: node:14.8.0
+    IMAGE_TAG_LATEST_FRONTEND: mimir-client:latest
+    IMAGE_TAG_LATEST_BACKEND: mimir-api:latest
+  
+stages:
+  - deploy
+
+# Cache node modules in between builds
+cache:
+  key: ${CI_COMMIT_REF_SLUG}
+  paths:
+    # backend dependencies
+    - $BACKEND_DIR/node_modules
+    # frontend dependencies
+    - $FRONTEND_DIR/node_modules
+
+###################################################################################################
+# Deploy stage jobs
+
+deploy backend:
+  stage: deploy
+  script:
+    - docker info
+    - docker build
+      -t $IMAGE_TAG_LATEST_BACKEND
+      -f $BACKEND_DIR/Dockerfile
+      ./$BACKEND_DIR
+    - docker image ls
+    - echo "$REGISTRY_PASSWORD" | docker login $REGISTRY_USER --username $CI_REGISTRY_USER --password-stdin
+    - docker tag $IMAGE_TAG_LATEST_BACKEND $REGISTRY_USER/$IMAGE_TAG_LATEST_BACKEND
+    - docker push $REGISTRY_USER/$IMAGE_TAG_LATEST_BACKEND
+
+  only:
+    changes:
+      - api/**
+      - api/**/*
+    refs:
+      - master
+
+deploy frontend:
+  stage: deploy
+  script:
+    - docker info
+    - docker build
+      -t $IMAGE_TAG_LATEST_FRONTEND
+      -f $FRONTEND_DIR/Dockerfile
+      ./$FRONTEND_DIR
+    - docker image ls
+    - echo "$REGISTRY_PASSWORD" | docker login $REGISTRY_USER --username $CI_REGISTRY_USER --password-stdin
+    - docker tag $IMAGE_TAG_LATEST_FRONTEND $REGISTRY_USER/$IMAGE_TAG_LATEST_FRONTEND
+    - docker push $REGISTRY_USER/$IMAGE_TAG_LATEST_FRONTEND
+  only:
+    changes:
+      - client/**
+      - client/**/*
+    refs:
+      - master