ISO installation failing due to missing firmware

Since last week new ISOs produced using the live build scripts appear to be missing firmware required for installation. The installer will ask for language and keyboard settings before getting stuck at the network hardware detection stage:

The ISO was built in accordance with the official documentation using the following command: $ ./build.sh --installer --verbose

According to the Debian installer log there are some missing packages, however it isn't clear if this is expected behaviour:

Skipping the network card selection section presents another missing firmware error when moving on to the storage and partitioning section.

As a result of these errors it is not possible to produce a working installer ISO.