Capture missing logs in second-stage conversion

For a while now, we didn't get logs for the second-stage conversion
when using containers. Extend the code to log any captured output from
the second stage conversion, only if we run Dangerzone via our dev
entrypoint.

Note that the Qubes isolation provider was always logging output from
the second stage of the conversion.

dangerzone/isolation_provider/container.py

@@ -12,7 +12,7 @@ from typing import Any, List, Optional
 from ..conversion import errors
 from ..document import Document
 from ..util import get_resource_path, get_subprocess_startupinfo, get_tmp_dir
-from .base import IsolationProvider
+from .base import PIXELS_TO_PDF_LOG_END, PIXELS_TO_PDF_LOG_START, IsolationProvider
 
 # Define startupinfo for subprocesses
 if platform.system() == "Windows":
@@ -227,6 +227,17 @@ class Container(IsolationProvider):
             for line in pixels_to_pdf_proc.stdout:
                 self.parse_progress_trusted(document, line.decode())
         error_code = pixels_to_pdf_proc.wait()
+
+        # In case of a dev run, log everything from the second container.
+        if getattr(sys, "dangerzone_dev", False):
+            assert pixels_to_pdf_proc.stderr
+            out = pixels_to_pdf_proc.stderr.read().decode()
+            text = (
+                f"Conversion output: (pixels to PDF)\n"
+                f"{PIXELS_TO_PDF_LOG_START}\n{out}\n{PIXELS_TO_PDF_LOG_END}"
+            )
+            log.info(text)
+
         if error_code != 0:
             log.error("pixels-to-pdf failed")
             raise errors.exception_from_error_code(error_code)