fix: package.json & package-lock.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-ANSIREGEX-1583908

.github/workflows/docker.yml

@@ -0,0 +1,34 @@
+name: Docker
+
+on: ["push", "pull_request"]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Cache Docker layers
+        uses: actions/cache@v2
+        with:
+          path: /tmp/.buildx-cache
+          key: ${{ runner.os }}-buildx-${{ github.sha }}
+          restore-keys: |
+            ${{ runner.os }}-buildx-
+      - name: Build
+        uses: docker/build-push-action@v2
+        with:
+          context: ./
+          file: ./Dockerfile
+          builder: ${{ steps.buildx.outputs.name }}
+          push: false
+          cache-from: type=local,src=/tmp/.buildx-cache
+          cache-to: type=local,dest=/tmp/.buildx-cache-new
+      - name: Move cache
+        run: |
+          rm -rf /tmp/.buildx-cache
+          mv /tmp/.buildx-cache-new /tmp/.buildx-cache
+      - name: Image digest
+        run: echo ${{ steps.docker_build.outputs.digest }}

package.json

@@ -14,9 +14,9 @@
     "ncp": "^2.0.0",
     "node-prismjs": "^0.1.0",
     "prismjs": "^1.23.0",
-    "redis": "^3.1.1",
+    "redis": "^3.0.2",
     "rss": "^1.2.2",
-    "showdown": "^1.9.1"
+    "showdown": "^2.0.2"
   },
   "devDependencies": {
     "coveralls": "^3.0.4",