osbuild: don't show log output on the terminal

The output makes it hard to see which stage is currently processed and
how to enter the build container. Also, it doesn't include all relevant
logs.

Instead, stream log output into /tmp/output in the build container. Keep
outputting it to stdout, so that osbuild can collect it in the future.

osbuild

@@ -86,7 +86,9 @@ def main(pipeline_path, from_archive, save, sit):
                                 f"--bind={os.getcwd()}/stages/{name}:/tmp/stage",
                                 "--bind=/etc/pki",
                                 "/tmp/run-stage", *opts, "/tmp/stage"],
-                    input=options_str, encoding="utf-8", check=True)
+                    input=options_str, encoding="utf-8",
+                    stdout=subprocess.DEVNULL, stderr=subprocess.STDOUT,
+                    check=True)
             except KeyboardInterrupt:
                 print()
                 print(f"{RESET}{BOLD}{RED}Aborted{RESET}")

run-stage

@@ -18,16 +18,24 @@ def main(stage, sit):
     # directories owned by users that are not set up with systemd-sysusers.
     subprocess.run(["systemd-tmpfiles", "--create"])
 
-    r = subprocess.run([stage])
+    proc = subprocess.Popen([stage], stdout=subprocess.PIPE, stderr=subprocess.STDOUT, encoding="utf-8")
 
-    if sit and r.returncode != 0:
+    with open("/tmp/output", "w") as output:
+        for line in proc.stdout:
+            sys.stdout.write(line)
+            output.write(line)
+            output.flush()
+
+    returncode = proc.wait()
+
+    if sit and returncode != 0:
         import signal
         print()
-        print(f"{RESET}{BOLD}{RED}Stage failed with code {r.returncode}{RESET}")
+        print(f"{RESET}{BOLD}{RED}Stage failed with code {returncode}{RESET}")
         print("Keeping the build environment running...")
         signal.pause()
 
-    return r.returncode
+    return returncode
 
 
 if __name__ == "__main__":