Add initial jq-based templating engine

This finally brings us better consistency between versions, and encodes the differences between the images in code instead of just as a delta that has to be viewed via `diff`.

Author: tianon

.gitattributes

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+**/Dockerfile            linguist-generated
+**/docker-entrypoint.sh  linguist-generated
+Dockerfile.template      linguist-language=Dockerfile

.github/workflows/verify-templating.yml

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+name: Verify Templating
+
+on:
+  pull_request:
+  push:
+
+defaults:
+  run:
+    shell: 'bash -Eeuo pipefail -x {0}'
+
+jobs:
+  apply-templates:
+    name: Check For Uncomitted Changes
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Apply Templates
+        run: ./apply-templates.sh
+      - name: Check Git Status
+        run: |
+          status="$(git status --short)"
+          [ -z "$status" ]

.gitignore

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+.jq-template.awk