Use the semi-official GitHub mirror for GStreamer

See comments in the change for details.

I can't find any reference to this repo from an official GStreamer
source, but I've always understood the "Public mirror" badge on GitHub
to be special, and not something anyone can put on their repos.

Besides my desire to shift the bandwidth and CPU burden of this shallow
Git clone from (presumably) a team of volunteers onto one of the world's
largest corporations, I figure it's faster and cheaper for a GitHub
Actions runner to clone from here than from an arbitrary place on the
Internet.

Dockerfile

@@ -8,6 +8,16 @@ ARG NODEJS_BASE=docker.io/library/node:22-alpine
 # See https://gstreamer.freedesktop.org/news/.
 ARG GSTREAMER_VERSION=1.24.9
 
+# Mirror of https://gitlab.freedesktop.org/gstreamer/gstreamer.git.
+# Since this is formally marked as a "public mirror" in the GitHub web UI, my
+# understanding is that GitHub (the entity I trust to host and build Hypcast
+# itself) does the mirroring. Cloning from Git and doing a Meson build from the
+# checkout isn't as good as building real ".apk" files from a source tarball.
+# But until I put in the work to make this GStreamer build more than a pile of
+# shameful hacks, I'd rather use Microsoft's bandwidth and resources than those
+# of the freedesktop.org maintainers.
+ARG GSTREAMER_REPO=https://github.com/GStreamer/gstreamer.git
+
 
 # Let's get the client build out of the way, since it's much simpler than
 # everything that follows.
@@ -54,9 +64,8 @@ RUN \
 # Does Cerbero support the level of build customization we're looking for?)
 FROM --platform=$BUILDPLATFORM $ALPINE_BASE AS gst-build-base
 RUN apk add --no-cache bash git clang lld llvm pkgconf meson flex bison glib-dev
-ARG GSTREAMER_VERSION
-RUN git clone -b $GSTREAMER_VERSION --depth 1 \
-  https://gitlab.freedesktop.org/gstreamer/gstreamer.git /tmp/gstreamer
+ARG GSTREAMER_REPO GSTREAMER_VERSION
+RUN git clone -b $GSTREAMER_VERSION --depth 1 $GSTREAMER_REPO /tmp/gstreamer
 WORKDIR /tmp/gstreamer
 COPY build/hypcast-buildenv.sh /hypcast-buildenv.sh
 COPY build/gstreamer-build.bash .