fix(GRO-682): pre-populate corepack cache at build time

corepack prepare now runs during Docker build (both builder and runner
stages) so the cache directory is populated before readOnlyRootFilesystem
is enforced at runtime. Previously the mkdir existed without populating
the cache, causing ENOENT errors in migrate/seed jobs.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/Dockerfile

@@ -12,7 +12,8 @@ RUN pnpm install --frozen-lockfile
 
 # Build
 FROM deps AS builder
-RUN mkdir -p /home/node/.cache/node/corepack
+RUN mkdir -p /home/node/.cache/node/corepack && \
+    corepack prepare pnpm@9.15.4 --activate
 COPY packages/ packages/
 COPY apps/api/ apps/api/
 RUN pnpm --filter @groombook/types build && \
@@ -21,7 +22,9 @@ RUN pnpm --filter @groombook/types build && \
 
 # Runtime
 FROM node:20-alpine AS runner
-RUN corepack enable && corepack prepare pnpm@9.15.4 --activate
+RUN corepack enable && \
+    mkdir -p /home/node/.cache/node/corepack && \
+    corepack prepare pnpm@9.15.4 --activate
 WORKDIR /app
 ENV NODE_ENV=production