xx-apt: print-source-file flag

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

README.md

@@ -134,6 +134,9 @@ ARG TARGETPLATFORM
 RUN xx-apt-get install -y libc6-dev zlib1g-dev
 ```
 
+> [!NOTE]
+> `xx-apt --print-source-file` can be used to print the path of the main [Apt sources configuration file](https://manpages.debian.org/bookworm/apt/sources.list.5.en.html)
+
 Installing two meta-libraries, `xx-c-essentials`, `xx-cxx-essentials` is also allowed that expand the minimum necessary packages for either base image.
 
 ## `xx-verify` - Verifying compilation results

src/test-apt.bats

@@ -219,3 +219,12 @@ load 'assert'
   run apt show wget-notexist
   assert_failure
 }
+
+@test "print-source-file" {
+  run xx-apt --print-source-file
+  assert_success
+  assert_output --partial "/etc/apt/sources.list"
+
+  run test -e "$(xx-apt --print-source-file)"
+  assert_success
+}

src/xx-apt

@@ -38,6 +38,20 @@ exitnolinux() {
   fi
 }
 
+n=$#
+for a in "$@"; do
+  if [ $# = $n ]; then set --; fi
+  case "$a" in
+    "--print-source-file")
+      aptsourcesfile
+      exit 0
+      ;;
+    *)
+      set -- "$@" "$a"
+      ;;
+  esac
+done
+
 # these are not supported yet
 case "${TARGETARCH}" in
   mips*)