fix(docker): install libspice-client-glib runtime lib so SPICE loads (#195)

The builder stage compiles the guacd SPICE plugin (--with-spice + the
-dev lib), but the runtime stage never installed the SPICE runtime
shared library. libguac-client-spice.so was present yet failed to
dlopen (libspice-client-glib-2.0.so.8 missing), so guacd reported
"Support for protocol spice is not installed". Add the runtime lib
to the runtime stage; apt pulls the rest of the chain (gstreamer,
opus). Bare-metal installs were unaffected. Broken since SPICE was
vendored (c2c5679).

Dockerfile

@@ -95,6 +95,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libcairo2 libjpeg62-turbo libpng16-16t64 libwebp7 \
     libssh2-1 libssl3t64 libvncclient1 \
     libpango-1.0-0 libpulse0 \
+    libspice-client-glib-2.0-8 \
     libavcodec61 libavformat61 libavutil59 libswscale8 \
     libtelnet2 libwebsockets19t64 \
     libfreerdp3-3 libfreerdp-client3-3 libwinpr3-3 \