add packages required for replaygain plugin

Dockerfile

@@ -18,6 +18,7 @@ RUN \
 	ffmpeg \
 	ffmpeg-libs \
 	gdbm \
+	gst-plugins-good1 \
 	gstreamer1 \
 	jpeg \
 	lame \
@@ -26,6 +27,7 @@ RUN \
 	nano \
 	openjpeg \
 	py-gobject \
+	py-gobject3 \
 	py-pip \
 	python \
 	py-unidecode \

README.md

@@ -83,6 +83,7 @@ Contains [beets-copyartifacts](https://github.com/sbarakat/beets-copyartifacts)
 
 ## Versions
 
++ **16.01.17:** Add packages required for replaygain.
 + **24.12.16:** Add [beets-copyartifacts](https://github.com/sbarakat/beets-copyartifacts) plugin.
 + **07.12.16:** Edit cmake options for chromaprint, should now build and install fpcalc, add gstreamer lib
 + **14.10.16:** Add version layer information.