Dockerfile definition and helper scripts related to preparing a toolchain image able to produce WASM binaries

Includes emscripten toolchain like also precompiled libraries:

• boost
• openssl
• sec256k1-zkp

scripts/emscripten/emscripten_toolset.patch

+diff --git a/src/tools/emscripten.jam b/src/tools/emscripten.jam
+index d6594c5e4..d7ab28ed1 100644
+--- a/src/tools/emscripten.jam
++++ b/src/tools/emscripten.jam
+@@ -6,6 +6,7 @@
+ import feature ;
+ import os ;
+ import toolset ;
++import generators ;
+ import common ;
+ import gcc ;
+ import type ;
+@@ -51,6 +52,9 @@ toolset.inherit-flags emscripten : gcc
+         <rtti>off <rtti>on
+         ;
+ 
++generators.override builtin.lib-generator : emscripten.prebuilt ;
++generators.override emscripten.searched-lib-generator : searched-lib-generator ;
++
+ type.set-generated-target-suffix EXE : <toolset>emscripten : "js" ;
+ type.set-generated-target-suffix OBJ : <toolset>emscripten : "bc" ;
+ type.set-generated-target-suffix STATIC_LIB : <toolset>emscripten : "bc" ;