Fix cron container: switch dcron→cron, export env vars for cron jobs

- dcron doesn't support /etc/cron.d/ (vixie-cron format) — replace with
  standard 'cron' (vixie cron) which does
- Cron jobs run with a stripped environment and couldn't reach the database;
  fix by dumping Docker env vars to /etc/environment at startup and sourcing
  it in the cron job entry

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile.cron

@@ -2,7 +2,7 @@ FROM python:3.12-slim
 
 WORKDIR /app
 
-RUN apt-get update && apt-get install -y --no-install-recommends dcron && rm -rf /var/lib/apt/lists/*
+RUN apt-get update && apt-get install -y --no-install-recommends cron && rm -rf /var/lib/apt/lists/*
 
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
@@ -10,9 +10,12 @@ RUN pip install --no-cache-dir -r requirements.txt
 COPY . .
 
 # Write cron job: refresh podcast feeds every hour
-RUN echo "0 * * * * root cd /app && python manage.py refresh_feeds >> /var/log/cron.log 2>&1" \
+# The job sources /etc/environment so Docker env vars (DB path, secret key etc.) are available.
+RUN echo "0 * * * * root . /etc/environment; cd /app && python manage.py refresh_feeds >> /var/log/cron.log 2>&1" \
     > /etc/cron.d/podcast-refresh && \
     chmod 0644 /etc/cron.d/podcast-refresh && \
     touch /var/log/cron.log
 
-CMD ["dcron", "-f"]
+# Dump Docker env vars into /etc/environment at container start so cron jobs can read them,
+# then launch vixie cron in foreground.
+CMD ["bash", "-c", "printenv > /etc/environment && cron -f"]