Revise manylinux arch warning and add removal note for manylinux imag…

…e change

changes/1564.removal.rst

@@ -0,0 +1 @@
+New projects will now use ``manylinux_2_28`` instead of ``manylinux2014`` to create AppImages in Docker.

src/briefcase/integrations/linuxdeploy.py

@@ -54,7 +54,11 @@ def arch(cls, tools: ToolCache) -> str:
         """The architecture defined (and supported) by linuxdeploy for AppImages."""
         system_arch = tools.host_arch
 
-        # use 32bit linuxdeploy if using 32bit Python on 64bit hardware
+        # If Python is 32 bit, then use 32 bit linuxdeploy regardless of hardware.
+        # It is non-trivial to determine if Linux is 32 bit or 64 bit; so, this uses
+        # Python's bitness as a proxy for Linux's bitness. Furthermore, though, pip
+        # will install 32 bit packages if Python is 32 bit. So, using 32 bit
+        # linuxdeploy in this case ensures the entire resulting AppImage is consistent.
         if tools.is_32bit_python:
             system_arch = {
                 "aarch64": "armv8l",

src/briefcase/platforms/linux/appimage.py

@@ -182,9 +182,9 @@ def output_format_template_context(self, app: AppConfig):
                 "aarch64": "aarch64",
             }[LinuxDeploy.arch(self.tools)]
         except KeyError:
-            manylinux_arch = "unknown"
+            manylinux_arch = LinuxDeploy.arch(self.tools)
             self.logger.warning(
-                f"There is not a manylinux base image for {LinuxDeploy.arch(self.tools)}"
+                f"There is no manylinux base image for {manylinux_arch}"
             )
 
         # Add the manylinux tag to the template context.