add type get_container_runtime & handle no runtime

There was no code to handle if at this stage the runtime existed.
This caused issues with type hints since `shutil.which()` can
return None, which had not previously been accounted for.

We did not use the opportunity to consolidate all the code for
detecting the runtime, to make this review easier.

dangerzone/container.py

@@ -52,11 +52,17 @@ def exec(args, stdout_callback=None):
 def exec_container(command, extra_args=[], stdout_callback=None):
     if container_tech == "podman":
         container_runtime = shutil.which("podman")
+        if container_runtime is None:
+            raise Exception(f"podman is not installed")
+
         platform_args = []
         security_args = ["--security-opt", "no-new-privileges"]
         security_args += ["--userns", "keep-id"]
     else:
         container_runtime = shutil.which("docker")
+        if container_runtime is None:
+            raise Exception(f"docker is not installed")
+
         platform_args = ["--platform", "linux/amd64"]
         security_args = ["--security-opt=no-new-privileges:true"]
 

dangerzone/global_common.py

@@ -386,11 +386,15 @@ class GlobalCommon(object):
         )
         print(Back.BLACK + Fore.YELLOW + Style.DIM + "╰──────────────────────────╯")
 
-    def get_container_runtime(self):
+    def get_container_runtime(self) -> str:
         if platform.system() == "Linux":
-            return shutil.which("podman")
+            runtime_name = "podman"
         else:
-            return shutil.which("docker")
+            runtime_name = "docker"
+        runtime = shutil.which(runtime_name)
+        if runtime is None:
+            raise Exception(f"{runtime_name} is not installed")
+        return runtime
 
     def get_resource_path(self, filename: str) -> str:
         if getattr(sys, "dangerzone_dev", False):