Add zstd decompression to launcher

To allow for handling compressed raw images, have the launcher
automatically decompress images with a .zst extension.

Change-Id: I00151aa487f7d09a9ef620a9abf61e7b21259a53

Dockerfile

@@ -116,6 +116,11 @@ FROM zuul as zuul-fingergw
 CMD ["/usr/local/bin/zuul-fingergw", "-f"]
 
 FROM zuul as zuul-launcher
+# Install zstd to decompress images
+RUN apt-get update \
+  && apt-get install -y zstd \
+  && apt-get clean \
+  && rm -rf /var/lib/apt/lists/*
 CMD ["/usr/local/bin/zuul-launcher", "-f"]
 
 FROM zuul as zuul-merger

zuul/launcher/server.py

@@ -21,6 +21,7 @@ import logging
 import os
 import random
 import socket
+import subprocess
 import threading
 import time
 import uuid
@@ -1066,6 +1067,11 @@ class Launcher:
             for future in concurrent.futures.as_completed(futures):
                 future.result()
         self.log.debug("Downloaded %s bytes to %s", size, path)
+        if path.endswith('.zst'):
+            subprocess.run(["zstd", "-dq", path],
+                           cwd=self.temp_dir, check=True, capture_output=True)
+            path = path[:-len('.zst')]
+            self.log.debug("Decompressed image to %s", path)
         return path
 
     def getImageExternalId(self, node, provider):