govulncheck reads vuln.go.dev directly, so on networks that only reach
the proxy it cannot run. upstream.go_vulndb (default https://vuln.go.dev)
is now served at /go/vulndb for `govulncheck -db`.
Only the files the database API defines are passed on (index/db,
index/modules, index/vulns and ID/{id}, as .json and .json.gz), with IDs
limited to letters, digits, '-' and '_'. They are stored and served
byte-for-byte; .json.gz files are gzip data without Content-Encoding.
All files, entries included, follow the metadata TTL since entries change
when their modified date does, and fall back to the cached copy when the
database is unreachable. Non-JSON responses are refused so a block page
does not replace the copy kept for an outage. HEAD is allowed under
/vulndb/, because govulncheck checks a non-default database with HEAD on
index/modules.json.gz.
