[pinguinfuss]

Fixes #497.

gradle.build_cache.read_only now defaults to true, so a client that can reach the proxy can't PUT to /gradle/ and overwrite cache entries that end up in other people's builds. GET and HEAD work as before.

This is a breaking change for setups that upload to the cache: set read_only: false (or PROXY_GRADLE_BUILD_CACHE_READ_ONLY=false) on the instance your CI jobs push to. docs/configuration.md describes this, and the README and dashboard snippets now push only from CI (isPush = System.getenv("CI") != null, Kotlin DSL).

PROXY_GRADLE_BUILD_CACHE_READ_ONLY is parsed strictly. The shared envBool treats anything other than true/1 as false, so with the new default a value like yes or TRUE would have opened uploads. The variable now accepts what strconv.ParseBool accepts; any other value keeps the cache read-only and logs a warning at startup. The other boolean variables are unchanged.

Tests cover the new default, a config file with a partial build_cache block, the environment values, and the server wiring (PUT returns 405 by default and 201 with read_only: false).

[andrew]

The read-only default, explicit upload opt-in, and strict environment parsing are covered by configuration and HTTP server tests. The configuration documentation explains the changed default for existing upload setups.