[Dockerfile] Fix creation of plugins.json with buildah (#1602)

Multi-line RUN instructions are unsupported by default docker (works 
with buildkit) and buildah, instead one has to use other tricks like 
ANSI-C quoting for the shell via \n\ and multiple echo commands.

Includes a buildah github workflow to help ensure future
compatibility.

.github/workflows/docker-image.yml

@@ -9,6 +9,18 @@ on:
   pull_request:
 
 jobs:
+  build-image-with-buildah:
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      -
+        name: Build with buildah
+        run: buildah bud .
+
   build-docker-image:
     runs-on: ubuntu-latest
     steps: