Give in-dyno addon containers unique names

[blsmth]

Problem

A client build failed in the prebuild phase with:

FTL Error error="Error response from daemon: Conflict. The container name \"/db\" is already in use by container \"6b84fe4b...\". You have to remove (or rename) that container to be able to reuse that name."

CodeBuild reuses the Docker daemon across builds. The in-dyno test addons (db, redis) were created with fixed container names, so a leftover container from a previous build collides on the next one.

Fix

Prefix the addon container names with the (unique) CodeBuild build ID — the same scheme already used for the buildx builder and the test container. The friendly db/redis names are preserved as network aliases, so other containers can still reach the addon by hostname (e.g. DATABASE_URL=postgres://...@db:5432/postgres).

RunContainer gains an aliases []string parameter; the postbuild test container passes nil (it's already uniquely named).

Testing

Wrote a throwaway integration test against a real Docker daemon:

• Reproduced the original bug — a second container named db on the same daemon fails with Conflict. The container name "/db" is already in use.
• Verified the fix — two builds sharing one daemon each start their db addon with unique names (<build-id>-db) and no conflict; the db network alias still resolves (getent hosts db → 172.19.0.2 db), so DATABASE_URL=...@db:5432 is unaffected.
• go test ./... passes.

Closes #27