Add support for mp3 demuxer/decoder.

Dockerfile

@@ -2,6 +2,7 @@ FROM emscripten/emsdk:3.1.15 as build
 
 ARG FFMPEG_VERSION=4.3.1
 ARG X264_VERSION=20170226-2245-stable
+ARG LAME_VERSION=3.100 
 
 ARG PREFIX=/opt/ffmpeg
 ARG MAKEFLAGS="-j4"
@@ -25,6 +26,21 @@ RUN cd /tmp/x264-snapshot-${X264_VERSION} && \
 RUN cd /tmp/x264-snapshot-${X264_VERSION} && \
   emmake make && emmake make install 
 
+# libmp3lame
+RUN cd /tmp && \
+  wget -O lame-${LAME_VERSION}.tar.gz https://sourceforge.net/projects/lame/files/lame/${LAME_VERSION}/lame-${LAME_VERSION}.tar.gz/download && \
+  tar zxf lame-${LAME_VERSION}.tar.gz
+
+RUN cd /tmp/lame-${LAME_VERSION} && \
+  emconfigure ./configure \
+  --prefix=${PREFIX} \
+  --host=i686-gnu \
+  --enable-static \
+  --disable-frontend
+
+RUN cd /tmp/lame-${LAME_VERSION} && \
+  emmake make && emmake make install 
+
 # Get ffmpeg source.
 RUN cd /tmp/ && \
   wget http://ffmpeg.org/releases/ffmpeg-${FFMPEG_VERSION}.tar.gz && \
@@ -56,11 +72,12 @@ RUN cd /tmp/ffmpeg-${FFMPEG_VERSION} && \
   --enable-postproc \
   --enable-swscale \
   --enable-protocol=file \
-  --enable-decoder=h264,aac,pcm_s16le \
-  --enable-demuxer=mov,matroska \
+  --enable-decoder=h264,aac,pcm_s16le,mp3 \
+  --enable-demuxer=mov,matroska,mp3 \
   --enable-muxer=mp4 \
   --enable-gpl \
   --enable-libx264 \
+  --enable-libmp3lame \
   --extra-cflags="$CFLAGS" \
   --extra-cxxflags="$CFLAGS" \
   --extra-ldflags="$LDFLAGS" \