fix(bash_header): PKG_INSTALL string → function + export DEBIAN_FRONTEND

The previous definition assigned a multi-word string to PKG_INSTALL:
  PKG_INSTALL="DEBIAN_FRONTEND=noninteractive apt-get install -y"
Downstream generators expand it as '$PKG_INSTALL pkg…', which bash
parses as 'DEBIAN_FRONTEND=noninteractive apt-get install -y pkg…'
— the leading assignment is treated as a command name, producing
'DEBIAN_FRONTEND=noninteractive: command not found' (e.g. docker.sh
line 40). The DEBIAN_FRONTEND variable was also never set, so apt
could fall into interactive mode for tzdata / debconf prompts.

Fix: turn PKG_INSTALL into a shell function and export
DEBIAN_FRONTEND separately. All ~50 call sites keep the existing
$PKG_INSTALL pkg… syntax unchanged.

Verified: bash -n OK; mock Ubuntu 22.04 run exits 0 with no
'command not found'; mock apt-get receives the correct args.
All 44 generators still render successfully.

generators/__init__.py

@@ -137,14 +137,27 @@ die()  {{ echo "${{LOG_PREFIX}} ERROR: $*" >&2 ; exit 1 ; }}
 DISTRO_ID="${{ID:-unknown}}"
 DISTRO_LIKE="${{ID_LIKE:-}}"
 case "$DISTRO_ID" in
-    ubuntu|debian) PKG="apt-get"; PKG_INSTALL="DEBIAN_FRONTEND=noninteractive apt-get install -y" ;;
+    ubuntu|debian)
-    centos|rhel|rocky|almalinux|ol) PKG="yum"; PKG_INSTALL="yum install -y" ;;
+        PKG="apt-get"
-    fedora) PKG="dnf"; PKG_INSTALL="dnf install -y" ;;
+        export DEBIAN_FRONTEND=noninteractive
+        PKG_INSTALL() {{ DEBIAN_FRONTEND=noninteractive apt-get install -y "$@"; }}
+        ;;
+    centos|rhel|rocky|almalinux|ol)
+        PKG="yum"
+        PKG_INSTALL() {{ yum install -y "$@"; }}
+        ;;
+    fedora)
+        PKG="dnf"
+        PKG_INSTALL() {{ dnf install -y "$@"; }}
+        ;;
     *)
         if echo "$DISTRO_LIKE" | grep -qiE "rhel|centos|fedora"; then
-            PKG="yum"; PKG_INSTALL="yum install -y"
+            PKG="yum"
+            PKG_INSTALL() {{ yum install -y "$@"; }}
         elif echo "$DISTRO_LIKE" | grep -qi "debian"; then
-            PKG="apt-get"; PKG_INSTALL="DEBIAN_FRONTEND=noninteractive apt-get install -y"
+            PKG="apt-get"
+            export DEBIAN_FRONTEND=noninteractive
+            PKG_INSTALL() {{ DEBIAN_FRONTEND=noninteractive apt-get install -y "$@"; }}
         else
             die "Unsupported distribution: $DISTRO_ID"
         fi