Add release notes for PR #7861

After #7602 moved the browser build to lage, the root sync-server.Dockerfile
broke in two ways:

1. lage.config.js was never copied into the deps stage, so `lage` had no
   pipeline config inside the image.
2. lage's task hasher invokes `git ls-tree HEAD` during initialization (even
   for targets with `cache: false`), but .dockerignore strips `.git`, so the
   build aborts before vite ever runs.

Copy lage.config.js alongside the other root config files, and seed a
throwaway single-commit git repo in the builder stage right before
`yarn build:server` so lage's hasher has a HEAD to read.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sync-server.Dockerfile

@@ -7,7 +7,7 @@ WORKDIR /app
 
 # Copy only the files needed for installing dependencies
 COPY .yarn ./.yarn
-COPY yarn.lock package.json .yarnrc.yml tsconfig.json ./
+COPY yarn.lock package.json .yarnrc.yml tsconfig.json lage.config.js ./
 COPY packages/api/package.json packages/api/package.json
 COPY packages/component-library/package.json packages/component-library/package.json
 COPY packages/crdt/package.json packages/crdt/package.json
@@ -31,6 +31,13 @@ COPY packages/ ./packages/
 # Increase memory limit for the build process to 8GB
 ENV NODE_OPTIONS=--max_old_space_size=8192
 
+# lage's task hasher invokes `git ls-tree HEAD` during initialization, so it
+# needs a git repo even when individual targets disable caching. .dockerignore
+# omits the real .git, so seed a throwaway repo with a single commit here.
+RUN git -c init.defaultBranch=master init -q \
+    && git -c user.email=build@docker -c user.name=docker-build add -A \
+    && git -c user.email=build@docker -c user.name=docker-build commit -qm build
+
 RUN yarn build:server
 
 # Focus the workspaces in production mode (including @actual-app/web you just built)

upcoming-release-notes/7861.md

@@ -0,0 +1,6 @@
+---
+category: Bugfixes
+authors: [MatissJanis]
+---
+
+Update Dockerfile to ensure `yarn build:server` works after lage migration.