[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:

Dockerfile

@@ -1,4 +1,4 @@
-# syntax = docker/dockerfile:1.3-labs
+# syntax = docker/dockerfile:1.0-experimental
 ARG VAGRANT_VERSION=2.3.0
 
 
@@ -38,8 +38,7 @@ FROM base as build
 # allow caching of packages for build
 RUN rm -f /etc/apt/apt.conf.d/docker-clean; echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
 RUN sed -i '/deb-src/s/^# //' /etc/apt/sources.list
-RUN --mount=id=apt-cache-v1,type=cache,target=/var/cache/apt --mount=id=apt-lib-v1,type=cache,target=/var/lib/apt \
+RUN apt update \
-    apt update \
     && apt build-dep -y \
         vagrant \
         ruby-libvirt \
@@ -61,20 +60,18 @@ RUN find /opt/vagrant/embedded/ -type f | grep -v /opt/vagrant/embedded/plugins.
 
 RUN /opt/vagrant/embedded/bin/gem install --install-dir /opt/vagrant/embedded/gems/${VAGRANT_VERSION} ./pkg/vagrant-libvirt*.gem
 
-RUN cat <<EOF > /opt/vagrant/embedded/plugins.json
+RUN echo -n '{\n\
-{
+    "version": "1",\n\
-    "version": "1",
+    "installed": {\n\
-    "installed": {
+        "vagrant-libvirt": {\n\
-        "vagrant-libvirt": {
+            "ruby_version": "'$(/opt/vagrant/embedded/bin/ruby -e 'puts "#{RUBY_VERSION}"')'",\n\
-            "ruby_version":"$(/opt/vagrant/embedded/bin/ruby -e 'puts RUBY_VERSION')",
+            "vagrant_version": "'${VAGRANT_VERSION}'",\n\
-            "vagrant_version":"${VAGRANT_VERSION}",
+            "gem_version":"",\n\
-            "gem_version":"",
+            "require":"",\n\
-            "require":"",
+            "sources":[]\n\
-            "sources":[]
+        }\n\
-        }
+    }\n\
-    }
+}' > /opt/vagrant/embedded/plugins.json
-}
-EOF
 
 FROM build as pruned