Adding test stages to Linux builds

* Build based on new OpenRGB docker image (Debian Stretch)
* Adapting AppImage build scripts to new docker image
* Changing postinstall script to avoid error

+ Adding test stage keyword
+ Adding needs keyword
+ Debian 10       Buster 32bit & 64bit
+ Debian 11       Bullseye 32bit & 64bit
+ Fedora 34       v34 64bit only
+ Ubuntu 18.04LTS Bionic 32bit & 64bit
+ Ubuntu 20.04LTS Focal 64bit only
+ Ubuntu 20.10    Groovy 64bit only
+ Mint   20.1     Ulyssa 32bit & 64bit

Amended to leave Bullseye builds in place by Adam Honse <calcprogrammer1@gmail.com>

filesystem.h

@@ -4,9 +4,21 @@
 #if defined(__has_include) && __has_include(<filesystem>)
 
 #include <filesystem>
-namespace filesystem = std::filesystem;
 
-#else
+// Debian 10 provides the header, but does not enable the feature, so we additionally check for the feature test macro
+// MSVC below 2017 does not provide feature test macros, so we leave an exception for them
+#if defined(__cpp_lib_filesystem) || defined (_MSC_VER)
+namespace filesystem = std::filesystem;
+#define STD_FILESYSTEM_FOUND
+#endif
+
+#endif
+
+#ifndef STD_FILESYSTEM_FOUND
+
+#if defined(__has_include) && !__has_include(<experimental/filesystem>)
+#error Neither <filesystem> nor <experimental/filesystem> could be found on the system! Please report an issue to OpenRGB developers providing your system info.
+#endif
 
 #define _SILENCE_EXPERIMENTAL_FILESYSTEM_DEPRECATION_WARNING
 #include <experimental/filesystem>