should work

Dockerfile

@@ -6,12 +6,12 @@ COPY requirements.txt .
 
 RUN pip install --no-cache-dir -r requirements.txt
 
-ENV CALDAV_URL 
-ENV CALENDAR_NAME 
-ENV USERNAME 
-ENV PASSWORD 
-ENV RA_API 
-ENV RA_USER 
+ENV CALDAV_URL $CALDAV_URL 
+ENV CALENDAR_NAME $CALENDAR_NAME 
+ENV USERNAME $USERNAME
+ENV PASSWORD $PASSWORD
+ENV RA_API $RA_API
+ENV RA_USER $RA_USER
 
 ENTRYPOINT ["sh", "-c"]
-CMD python3 ./app.py "${CALDAV_URL}" "${CALENDAR_NAME}" "${USERNAME}" "${PASSWORD}" "${RA_API}" "${RA_USER}"
+CMD python3 ./app.py "$CALDAV_URL" "$CALENDAR_NAME" "$USERNAME" "$PASSWORD" "$RA_API" "$RA_USER"