Add "libgdbm-compat-dev" explicitly in Buster+ on Slim variants

This makes "require 'dbm'" work -- it uses "ndbm.h" to detect support (which is provided by this package in Buster+).

2.7-rc/buster/slim/Dockerfile

@@ -38,6 +38,7 @@ RUN set -eux; \
 		dpkg-dev \
 		gcc \
 		libbz2-dev \
+		libgdbm-compat-dev \
 		libgdbm-dev \
 		libglib2.0-dev \
 		libncurses-dev \

Dockerfile-slim.template

@@ -39,6 +39,7 @@ RUN set -eux; \
 		dpkg-dev \
 		gcc \
 		libbz2-dev \
+		libgdbm-compat-dev \
 		libgdbm-dev \
 		libglib2.0-dev \
 		libncurses-dev \

update.sh

@@ -98,6 +98,13 @@ for version in "${versions[@]}"; do
 			-e 's/^(FROM (debian|buildpack-deps|alpine)):.*/\1:'"$tag"'/' \
 			"$template" > "$dir/Dockerfile"
 
+		case "$v" in
+			# https://packages.debian.org/sid/libgdbm-compat-dev (needed for "dbm" core module, but only in Buster+)
+			jessie/slim | stretch/slim)
+				sed -i -e '/libgdbm-compat-dev/d' "$dir/Dockerfile"
+				;;
+		esac
+
 		if [ -n "${newEnoughRubygems[$rcVersion]:-}" ]; then
 			sed -ri -e '/RUBYGEMS_VERSION/d' "$dir/Dockerfile"
 		fi