Add pre-commit (app) hook to check IMGSFX

Intended for use by [the pre-commit
app](https://pre-commit.com/#intro), this hook keeps track of all IMG_SFX
values pushed, failing when any duplicate is found.  In the case of
pushing to PRs that don't build CI VM images, the hook failure must be
manually bypassed.  Example `.pre-commit-config.yaml`:

```yaml
---
repos:
  - repo: https://github.com/containers/automation_images.git
    rev: <tag or commit sha>
    hooks:
      - id: check-imgsfx
```

Signed-off-by: Chris Evich <cevich@redhat.com>

.pre-commit-hooks.yaml

@@ -0,0 +1,20 @@
+---
+
+# Ref: https://pre-commit.com/#creating-new-hooks
+- id: check-imgsfx
+  name: Check IMG_SFX for accidental reuse.
+  description: |
+    Every PR intended to produce CI VM or container images must update
+    the `IMG_SFX` file via `make IMG_SFX`.  The exact value will be
+    validated against global suffix usage (encoded as tags on the
+    `imgts` container image).  This pre-commit hook verifies on every
+    push, the IMG_SFX file's value has not been pushed previously.
+    It's intended as a simple/imperfect way to save developers time
+    by avoiding force-pushes that will most certainly fail validation.
+  entry: ./check-imgsfx.sh
+  language: system
+  exclude: '.*'  # Not examining any specific file/dir/link
+  always_run: true  # ignore no matching files
+  fail_fast: true
+  pass_filenames: false
+  stages: ["pre-push"]

check-imgsfx.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+#
+# 2024-01-25 esm
+# 2024-06-28 cevich
+
+set -eo pipefail
+
+# Ensure CWD is the repo root
+cd $(dirname "${BASH_SOURCE[0]}")
+imgsfx=$(<IMG_SFX)
+
+if [[ -z "$HOME" ]]; then
+    echo "FATAL: $$HOME is undefined" >&2
+    exit 1
+fi
+
+# Pre-commit runs in a temp. dir, history must persist via absolute path.
+imgsfx_history="$HOME/.cache/pre-commit/imgsfx.history"
+
+if [[ -e $imgsfx_history ]]; then
+    if grep -q "$imgsfx" $imgsfx_history; then
+        echo "FATAL: $imgsfx has already been used" >&2
+        echo "Please rerun 'make IMG_SFX'" >&2
+        exit 1
+    fi
+fi
+
+mkdir -p $(dirname "$imgsfx_history")
+echo $imgsfx >>$imgsfx_history