chore: Speed up docker builds a bit by reducing layer count.

This is especially noticeable on local builds (less so on the github
workers).

other/docker/cppcheck/cppcheck.Dockerfile

@@ -0,0 +1,23 @@
+FROM toxchat/c-toxcore:sources AS sources
+FROM alpine:3.19.0
+
+RUN ["apk", "add", "--no-cache", \
+ "bash", \
+ "cppcheck", \
+ "findutils", \
+ "libconfig-dev", \
+ "libsodium-dev", \
+ "libvpx-dev", \
+ "linux-headers", \
+ "make", \
+ "opus-dev"]
+
+COPY --from=sources /src/ /src/workspace/c-toxcore/
+COPY other/analysis/run-cppcheck \
+     other/analysis/gen-file.sh \
+     other/analysis/variants.sh \
+     /src/workspace/c-toxcore/other/analysis/
+COPY other/docker/cppcheck/toxcore.cfg \
+     /src/workspace/c-toxcore/other/docker/cppcheck/
+WORKDIR /src/workspace/c-toxcore
+RUN ["other/analysis/run-cppcheck"]