Each `go mod download` in the Dockerfile now runs under a five-minute
time limit and is tried up to three times. Without a limit, a stalled
connection to the module proxy held the step, and every build on the
host sharing it, until the kernel dropped the connection. Five minutes
is well above the three minutes a working download can take on a busy
build host. When all three attempts fail, the step prints "module
download failed" and exits 1, so the build stops there instead of
failing as a test or lint failure. A test checks every download step
for the limit, the retries, the message and the final `exit 1`.
Model: opus-5-5
sneak