This adds 1k of weight to the resulting binary, which is reasonable
(it's less than 5% for the smaller non-static binary), but alleviates
legitimate user concern that the license requires being included when
Tini is redistributed.
The GPG signing subkey and passphrase are respectively provided through
a Travis encrypted file and a Travis encrypted environment variable.
Signing is only done if there is a signing key present when the build is
complete (so as to not fail when e.g. building a PR that doesn't have
encrypted files available).
Using the child subreaper mechanism, we can actually run
tests inside the CI environment without depending on Docker.
While this does not replace the existing tests, it allows
at least some functional coverage within CI.