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

.gitlab-ci.yml

@@ -161,11 +161,11 @@ build_tox_test_runner_image:
       exists:
         - Dockerfile.tox-test-runner
 
-build_emscripten_image:
+build_emsdk_image:
   extends: .build_docker_image
   stage: build
   variables:
-    BUILD_TARGET: "emscripten"
+    BUILD_TARGET: "emsdk"
   needs:
     - build_docker_dind_image
   rules: