Allow generating snapshot for any cpanfile

by adding optional parameter `path/to/cpanfile` to the script
`generate_cpanfile_snapshot.sh`.  Without parameter, it still
generates snapshot for the cpanfile in its grand-parent directory.
The script can still be called from any current working directory.

docker/generate_cpanfile_snapshot.sh

@@ -2,7 +2,13 @@
 
 set -e
 
+if [ $# -ne 0 -a "(" $# -ne 1 -o "${1##*/}" != "cpanfile" ")" ]; then
+    echo "Usage: $0 [path/to/cpanfile]"
+    exit 1
+fi
+
 MB_SERVER_ROOT=$(cd "$(dirname "${BASH_SOURCE[0]}")/../" && pwd)
+CPANFILE_DIR=$(cd "$(dirname ${1:-$MB_SERVER_ROOT/cpanfile})/" && pwd)
 cd "$MB_SERVER_ROOT"
 
 make -C docker config
@@ -13,14 +19,14 @@ TMP_DIR=/tmp/.$TMP_IMG
 mkdir -p "$TMP_DIR"
 cd "$TMP_DIR"
 
-cp "$MB_SERVER_ROOT/cpanfile" "$TMP_DIR/"
+cp "$CPANFILE_DIR/cpanfile" "$TMP_DIR/"
 cp "$MB_SERVER_ROOT/docker/Dockerfile.cpanfile-snapshot" "$TMP_DIR/Dockerfile"
 
 docker build -t $TMP_IMG .
 CONTAINER_ID=$(docker create $TMP_IMG)
 docker cp \
     $CONTAINER_ID:/home/musicbrainz/musicbrainz-server/cpanfile.snapshot \
-    "$MB_SERVER_ROOT/"
+    "$CPANFILE_DIR/"
 docker rm $CONTAINER_ID
 docker rmi $TMP_IMG