archive_dependencies.sh: Handle no dependencies (#463)

If there are no dependencies utilised that we need to archive source for,
this breaks the build process since no dependencies-src.zip file is
created, and the Dockerfile has nothing to copy.

This changes the script such that it starts with an empty zip file, and
then appends dependencies if and when they exist.

Unblocks #457

image/archive_dependencies.sh

@@ -26,15 +26,19 @@ CARGO_HOME="${CARGO_HOME:-$HOME/.cargo}"
 # This should be reviewed before each release to make sure we're capturing all
 # the dependencies we need.
 
-rm dependencies-src.zip || true
-
 dependencies=("slog-json")
 
 zip="$(pwd)/dependencies-src.zip"
 
+rm "$zip" || true
+
 echo "Archiving dependencies to: $zip"
+
+# create an empty zip, so we always have a file, even if we don't have dependencies
+echo UEsFBgAAAAAAAAAAAAAAAAAAAAAAAA== | base64 -d > "$zip"
+
 pushd "$CARGO_HOME/registry/src"
 for d in "${dependencies[@]}"; do
-  find . -type d -name "$d-*" | xargs -I {} zip -rv "$zip" "{}"
+  find . -type d -name "$d-*" | xargs -I {} zip -ruv "$zip" "{}"
 done
 popd