Fix run.dockerfile to actually be able to run current Servo

… and avoid runtime errors about missing gstreamer plugins

etc/taskcluster/docker/base.dockerfile

@@ -19,8 +19,12 @@ RUN \
         python-pip \
         python-dev \
         #
+        # Compiling C modules when installing Python packages in a virtualenv
+        gcc \
+        #
         # Installing rustup and sccache (build dockerfile) or fetching build artifacts (run tasks)
-        curl && \
+        curl \
+    && \
     # Running mach
     pip install virtualenv
 

etc/taskcluster/docker/run.dockerfile

@@ -1,10 +1,15 @@
 % include base.dockerfile
 
-# Servo’s runtime dependencies
+# Servo’s runtime dependencies:
 RUN apt-get install -qy --no-install-recommends \
     libgl1 \
     libssl1.1 \
     libdbus-1-3 \
-    libgstreamer-plugins-bad1.0-0 \
-    gstreamer1.0-plugins-good
-
+    gstreamer1.0-plugins-good \
+    gstreamer1.0-plugins-bad \
+    gstreamer1.0-libav \
+    gstreamer1.0-gl \
+    libunwind8 \
+    #
+    # Compiling shaders in OSMesa:
+    llvm \