[bug] docker generator should ouput logs as command is running

[jul-sh]

Describe the bug
The docker generator only outputs logs once the build command has returned, see

slsa-github-generator/internal/builders/docker/pkg/builder.go

Line 484 in 4534a0b

bytes, err := io.ReadAll(reader)

. This creates a problem when attempting to debug a process that hangs in CI, never succeeding and never failing.

Expected behavior
For verbose output, the generator should print logs as they are created by the build command.