Flatpak support (#4383)

* Initial flatpak support

* Fix compatibility list directory

* Hard-code SSH mount location

* Add workaround documentation

* Change SSH repo directory

* Change SSH repo directory (again)

* Fix variable name

* Remove temporary testing branch placeholder

* Use a flatpak-specific docker image

* Enable network access during the flatpak build so we can download compatibility list the right way

* Fix flatpak tag support

* Fix typo

* Use cloned git for the build

* Change SSH repo location

* Disable shallow git cloning, needed for tagged building

.travis/linux-flatpak/finish.sh

@@ -0,0 +1,9 @@
+#!/bin/bash -ex
+
+CITRA_SRC_DIR="/citra"
+REPO_DIR="$CITRA_SRC_DIR/repo"
+
+# When the script finishes, unmount the repository and delete sensitive files,
+# regardless of whether the build passes or fails
+umount "$REPO_DIR"
+rm -rf "$REPO_DIR" "/tmp/*"