CI: make Yarn Classic fallback cache a tmpfs (fresh per build)

#74 corrupted the persistent fallback cache again despite serialising the
fetch, so the cause isn't a write race: BuildKit evicts part of that persistent
cache mount between builds (the first build after each id bump always passed,
later ones failed). Mount /usr/local/share/.cache/yarn as tmpfs so it's clean
every build and nothing can be half-evicted; the Berry cache stays persistent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server-ce/Dockerfile

@@ -25,14 +25,18 @@ ENV COREPACK_ENABLE_NETWORK=0
 
 # Install yarn dependencies
 # -------------------------
-# YARN_NETWORK_CONCURRENCY=1 serialises Yarn's fetch step. The git-sourced
-# @replit/codemirror-* deps are prepared with Yarn Classic, which pulls *all*
-# esbuild platform binaries into the one shared /usr/local/share/.cache/yarn
-# folder; preparing several of them in parallel races and corrupts that cache
-# (EEXIST / missing .yarn-tarball.tgz). Serialising the fetch removes the race.
+# The git-sourced @replit/codemirror-* deps are prepared with Yarn Classic,
+# whose cache lives in /usr/local/share/.cache/yarn. We mount that as a *tmpfs*
+# (fresh every build) rather than a persistent BuildKit cache: when it was
+# persistent, BuildKit would garbage-collect/evict part of it between builds,
+# leaving a half-populated cache that Yarn Classic then tripped over (missing
+# .yarn-tarball.tgz / EEXIST). A clean cache per build is reliable; the cost is
+# re-fetching that small set of git deps. The valuable Berry cache
+# (server-ce-yarn-cache) stays persistent. YARN_NETWORK_CONCURRENCY=1 is kept
+# as cheap insurance against concurrent writes to the fresh cache.
 RUN --mount=type=cache,target=/root/.cache \
     --mount=type=cache,target=/root/.yarn/berry/cache,id=server-ce-yarn-cache \
-    --mount=type=cache,target=/usr/local/share/.cache/yarn,id=server-ce-yarn-fallback-cache-v3 \
+    --mount=type=tmpfs,target=/usr/local/share/.cache/yarn \
     --mount=type=tmpfs,target=/tmp node genScript install | YARN_NETWORK_CONCURRENCY=1 bash
 
 # Add the actual source files
@@ -40,7 +44,7 @@ RUN --mount=type=cache,target=/root/.cache \
 COPY --parents libraries/ services/ tools/migrations/ /overleaf/
 RUN --mount=type=cache,target=/root/.cache \
     --mount=type=cache,target=/root/.yarn/berry/cache,id=server-ce-yarn-cache \
-    --mount=type=cache,target=/usr/local/share/.cache/yarn,id=server-ce-yarn-fallback-cache-v3 \
+    --mount=type=tmpfs,target=/usr/local/share/.cache/yarn \
     --mount=type=cache,target=/overleaf/services/web/node_modules/.cache,id=server-ce-webpack-cache \
     --mount=type=tmpfs,target=/tmp \
     node genScript compile | YARN_NETWORK_CONCURRENCY=1 bash || \