Fix broken background image 
Fix broken background image 

This was getting linked directly, rather than with the digest hash, as for other assets.  Eg,

- `image-url("expand-collapse-transparent.png")` => https://assets.digital.cabinet-office.gov.uk/tariff/expand-collapse-transparent-8e508b555567c0618975a4a2db35d2571b3772a6e69303f97a4bc4866e365e8c.png
- `url("commodity-indents.png")` => https://assets.digital.cabinet-office.gov.uk/tariff/commodity-indents.png
bitzesty/trade-tariff-frontend,bitzesty/trade-tariff-frontend,alphagov/trade-tariff-frontend,alphagov/trade-tariff-frontend,alphagov/trade-tariff-frontend,bitzesty/trade-tariff-frontend,bitzesty/trade-tariff-frontend,alphagov/trade-tariff-frontend