feat: make amd64 and arm64 image build targets (E00-S02-T07)

compose.yaml

@@ -1,4 +1,4 @@
-# EPPP Docker Compose baseline — [E00-S02-T01..T06]
+# EPPP Docker Compose baseline — [E00-S02-T01..T07]
 #
 # `docker compose up -d` starts both the database (PostgreSQL) and the
 # application (@personal-blog/server). Rollback: `docker compose down`.
@@ -30,8 +30,15 @@
 # the only writable path. Rollback: drop `read_only`/`tmpfs` from the `app`
 # service.
 #
-# Explicitly out of scope for T01..T06 (land in later E00-S02 tasks):
-#   - multi-arch build targets (T07), secrets not embedded (T08)
+# Multi-arch build targets (T07): the `app` service's `build.platforms` list
+# declares `linux/amd64` and `linux/arm64` (Compose Build spec `platforms`), so
+# `docker compose build` produces a multi-platform image for both
+# architectures. `docker compose up` still builds and runs the host platform,
+# so local runs and the T01..T06 real-stack probes are unaffected. Rollback:
+# drop the `platforms` list from the `app` build config.
+#
+# Explicitly out of scope for T01..T07 (land in a later E00-S02 task):
+#   - secrets not embedded (T08)
 #
 # All values have defaults so `docker compose up -d` works from a clean clone
 # without a .env file (a committed .env.example template lands in E00-S04).
@@ -66,6 +73,13 @@ services:
     build:
       context: .
       dockerfile: apps/server/Dockerfile
+      # T07: multi-arch build targets — `docker compose build` produces a
+      # multi-platform image for linux/amd64 and linux/arm64 (Compose Build
+      # spec `platforms`). `docker compose up` builds/runs the host platform,
+      # so the T01..T06 real-stack probes are unaffected.
+      platforms:
+        - linux/amd64
+        - linux/arm64
     environment:
       DATABASE_URL: postgres://eppp:eppp@db:5432/eppp
     ports: