At run time the warm steps rebuilt the crates with build scripts on
every start, which took about two minutes. Cargo now builds only what a
command needs, when it runs.
On an incremental image build, setting a changed file to the commit
time can make it older than its stale build output, so cargo would skip
the rebuild. Rounding down to the whole second still survives the image
layer and never moves a file back past its outputs.
Pin source mtimes to the commit time at image build, so cargo's
fingerprints survive the image layer and the warm build stays fresh.
Build with line-tables-only debug info and drop the incremental cache
to shrink the baked target/.
Georgios Jason Efstathiou