Add builder-scripts/target-arch-alternative

Get the alternative name for the TARGETARCH for downloading pre-built
programs that use the alternative name. Adding in preparation for
downloading NCBI BLAST that uses `x86_64` and `aarch64`.

Unclear to me if there's a better way of doing this, but it works!

Author: joverlee521

builder-scripts/target-arch-alternative

@@ -0,0 +1,25 @@
+#!/bin/bash
+#
+# Get the alternative target arch name from a Docker target arch.
+#
+# The Docker target arch is provided by the TARGETARCH variable¹.
+# ¹ https://docs.docker.com/engine/reference/builder/#automatic-platform-args-in-the-global-scope
+#
+set -euo pipefail
+
+if [[ $# -gt 0 ]]; then
+    TARGETARCH="$1"
+fi
+
+case $TARGETARCH in
+    "amd64")
+        echo "x86_64"
+        ;;
+    "arm64")
+        echo "aarch64"
+        ;;
+    *)
+        echo "No target arch mapping for $TARGETARCH." 1>&2
+        exit 1
+        ;;
+esac