[servo] fix image build by installing a more recent gstreamer

Author: jankeromnes

servo/servo.dockerfile

@@ -1,5 +1,4 @@
 FROM janitortechnology/ubuntu-dev
-MAINTAINER Jan Keromnes "janx@linux.com"
 
 # Install Servo build dependencies.
 # Packages are from https://github.com/servo/servo/blob/master/README.md#on-debian-based-linuxes
@@ -63,6 +62,9 @@ RUN echo "\n# Work around https://github.com/servo/servo/issues/20712." >> /home
  && echo "export HARFBUZZ_SYS_NO_PKG_CONFIG=1" >> /home/user/.bashrc
 ENV HARFBUZZ_SYS_NO_PKG_CONFIG 1
 
+# Install a more recent GStreamer.
+RUN ./mach bootstrap-gstreamer
+
 # Build Servo.
 RUN ./mach build -d