Alpine base image ships no timezone data, so Date/moment() inside the
container fell back to UTC regardless of the host's real timezone -
every displayed timestamp (transaction list "Thời gian" column, CSV
export) and every signed timeStamp (epay merchantToken) was 7 hours
behind actual Vietnam time. Installs tzdata and sets TZ in both the
Dockerfile and docker-compose.yml (belt and suspenders - either alone
would fix it, but the compose env var makes it visible without having
to read the Dockerfile).

Verified by building the actual image and running new Date()/moment()
inside it: correctly shows GMT+0700 (Indochina Time) after the fix,
was UTC before.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017KPzWwuTEeX2vXGXvyGn4q