Install gpg2 and forward gpg-agent

For signing git commits, git uses GnuPG as an external program, defaulting to `gpg`. That binary is provided in the container, the problem with it is, it cannot use the gpg-agent as provided by Seahorse etc. and bails out on needing to supply a passphrase without a terminal.

The gpg program name can be overridden to `gpg2` in git configuration, but this binary is not present in the container. Neither, I believe, the agent socket is forwarded from the desktop.

首先检查容器的构建/配置以及 Git 的 GnuPG 设置处理方式；该 issue 未指定任何文件或测试。确认 gpg2 的安装方式，以及如何公开桌面 gpg-agent 的 socket。完成的标准是：无需交互式终端即可使用主机 agent 完成 Git commit 签名。

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