Update Dockerfile used in tests as well

This ensures the Dockerfile used for testing is making use of the
same scratch image to compile `libgit2` as the actual application
image.

In a future iteration we should restructure our GitHub Action
workflows to re-use the application image, saving us an additional
Dockerfile and a duplicate build. Inspiration for this (which makes
use of a local registry for the duration of the build) can be found
at: https://github.com/fluxcd/golang-with-libgit2/blob/main/.github/workflows/build.yaml

Signed-off-by: Hidde Beydals <hello@hidde.co>

.github/actions/run-tests/Dockerfile

@@ -1,4 +1,26 @@
-FROM ghcr.io/hiddeco/golang-with-libgit2:dev as build
+ARG BASE_VARIANT=bullseye
+ARG GO_VERSION=1.16.8
+ARG XX_VERSION=1.0.0-rc.2
+
+ARG LIBGIT2_IMG=ghcr.io/fluxcd/golang-with-libgit2
+ARG LIBGIT2_TAG=libgit2-1.1.1
+
+FROM tonistiigi/xx:${XX_VERSION} AS xx
+FROM ${LIBGIT2_IMG}:${LIBGIT2_TAG} as libgit2
+
+FROM golang:${GO_VERSION}-${BASE_VARIANT} as gostable
+
+# Copy the build utiltiies
+COPY --from=xx / /
+COPY --from=libgit2 /Makefile /libgit2/
+
+# Install the libgit2 build dependencies
+RUN make -C /libgit2 cmake
+
+RUN make -C /libgit2 dependencies
+
+# Compile and install libgit2
+RUN FLAGS=$(xx-clang --print-cmake-defines) make -C /libgit2 libgit2
 
 # Use the GitHub Actions uid:gid combination for proper fs permissions
 RUN groupadd -g 116 test && \