Changes Emscripten SDK version to match that used to compile the default export
templates used by Godot 3.4 (in this case 2.0.25 as can be seen here:
https://github.com/godotengine/build-containers/blob/3.4/Dockerfile.javascript).
This means we no longer need to compile custom export templates when
exporting HTML5 for Godot 3.4. Exports from other Godot versions may
requiring compiling the GDNative library with a different version of the
Emscripten SDK.
Adds support for building Linux binaries inside a docker container in
order to target an older version of GLIBC.
Updates GitHub Actions workflow to use it.
As a result the minimum version of GLIBC that Linux users need to have
installed on their system is 2.17 which was released in 2012.
