Add docker image with emscripten (#372)

skiko/docker/linux-emscripten-amd64/Dockerfile

+FROM ubuntu:focal-20211006
+SHELL ["/bin/bash", "-c", "-l"]
+ARG DEBIAN_FRONTEND=noninteractive
+RUN apt-get update -y && \
+    apt-get install binutils build-essential software-properties-common -y && \
+    apt-get install git curl wget -y && \
+    apt-get install python -y && \
+    apt-get install openjdk-11-jdk -y
+RUN export EMSDK_DIR=/usr/emsdk && \
+    export EMSDK_VER=2.0.29 && \
+    git clone https://github.com/emscripten-core/emsdk.git $EMSDK_DIR && \
+    $EMSDK_DIR/emsdk install $EMSDK_VER && \
+    $EMSDK_DIR/emsdk activate $EMSDK_VER && \
+    echo "source $EMSDK_DIR/emsdk_env.sh" >> ~/.bashrc
+ENV LANG=en_US.UTF-8 LANGUAGE=en_US:en LC_ALL=en_US.UTF-8
+ENV JAVA_TOOL_OPTIONS=-Dfile.encoding=UTF

skiko/docker/linux-emscripten-amd64/build.sh

+docker build -t skiko-build-emscripten-ubuntu-2004-amd64:latest .
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