Use Debian Slim image in GitHub action

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

Author: hiddeco

.github/actions/run-tests/Dockerfile

@@ -1,13 +1,23 @@
-FROM golang:1.16-alpine
+FROM golang:1.16-buster as builder
 
-# Add any build or testing essential system packages
-RUN apk add --no-cache build-base git pkgconf
-RUN apk add --no-cache --repository http://dl-cdn.alpinelinux.org/alpine/edge/community libgit2-dev~=1.1
+# Up-to-date libgit2 dependencies are only available in
+# unstable, as libssh2 in testing/bullseye has been linked
+# against gcrypt which causes issues with PKCS* formats.
+# Ref: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=668271
+RUN echo "deb http://deb.debian.org/debian unstable main" >> /etc/apt/sources.list \
+    && echo "deb-src http://deb.debian.org/debian unstable main" >> /etc/apt/sources.list
+RUN set -eux; \
+    apt-get update \
+    && apt-get install -y libgit2-dev/unstable \
+    && apt-get clean \
+    && apt-get autoremove --purge -y \
+    && rm -rf /var/lib/apt/lists/*
 
 # Use the GitHub Actions uid:gid combination for proper fs permissions
-RUN addgroup -g 116 -S test && adduser -u 1001 -S -g test test
+RUN groupadd -g 116 test && \
+    useradd -u 1001 --gid test --shell /bin/sh --create-home test
 
 # Run as test user
 USER test
 
-ENTRYPOINT ["/bin/sh", "-c"]
+ENTRYPOINT [ "/bin/sh", "-c" ]