Adds workflow to run shellcheck

Lints shell scripts using
[shellcheck](https://github.com/koalaman/shellcheck). There are other
options out there for github actions that run shellcheck and some appear
more configurable, but `bewuethr/shellcheck-action@v2` was the first one
I found that worked.

Author: ornj

.github/workflows/main.yml

@@ -0,0 +1,13 @@
+---
+on:
+  - push
+  - pull_request
+
+jobs:
+  shellcheck:
+    name: ShellCheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Run ShellCheck
+        uses: bewuethr/shellcheck-action@v2

src/deploy-config.sh

@@ -16,7 +16,7 @@ else
     exit 2
 fi
 
-encodedConfigContent=$(base64 $nginxConfigurationFile)
+encodedConfigContent=$(base64 "$nginxConfigurationFile")
 echo "Base64 encoded NGINX configuration content"
 echo "$encodedConfigContent"
 echo ""