fix: preserve credential lineage in dogfood container (#1487)

docker/gateway.Dockerfile

@@ -33,7 +33,6 @@ ENV NODE_ENV=production
 # bash/curl/python3 are runtime dependencies of the provider-neutral Mosaic git
 # wrappers. jq supports wrapper discovery for non-canonical Gitea hosts.
 RUN apk add --no-cache bash curl git jq python3 \
-  && ln -sf /bin/bash /usr/bin/bash \
   && mkdir -p /opt/mosaic/.workspaces \
   && chown -R node:node /opt/mosaic /app
 ENV MOSAIC_ROOT=/opt/mosaic
@@ -45,6 +44,11 @@ COPY --from=builder /app/packages/mosaic/framework/tools/git/ci-queue-wait.sh /o
 COPY --from=builder /app/packages/mosaic/framework/tools/git/detect-platform.sh /opt/mosaic/tools/git/detect-platform.sh
 COPY --from=builder /app/packages/mosaic/framework/tools/git/repo-decl.sh /opt/mosaic/tools/git/repo-decl.sh
 COPY --from=builder /app/packages/mosaic/framework/tools/git/git-credential-mosaic /opt/mosaic/tools/git/git-credential-mosaic
+# R4 hardening (P0-SEC, brain 15f6979a): the credential helper is a pair.
+# python entrypoint (allowlist envp, execve boundary) + the bash implementation
+# it execs. The entrypoint derives the .impl path from its own directory, so the
+# pair sits side by side; system gitconfig keeps pointing at the entrypoint.
+COPY --from=builder /app/packages/mosaic/framework/tools/git/git-credential-mosaic.impl /opt/mosaic/tools/git/git-credential-mosaic.impl
 COPY --from=builder /app/packages/mosaic/framework/tools/_lib/credentials.sh /opt/mosaic/tools/_lib/credentials.sh
 COPY --from=builder /app/packages/mosaic/framework/tools/structure/validate-repo-json.sh /opt/mosaic/tools/structure/validate-repo-json.sh
 RUN git config --system credential.helper /opt/mosaic/tools/git/git-credential-mosaic