这是预期的行为，看起来很奇怪！

This is the expected behavior, as weird as it seems!

当 ENTRYPOINT 是列表时（如 ENTRYPOINT [ echo， $ USER] ），按原样使用，无需进一步解析或解释。因此， $ USER 仍然是 $ USER ，因为在此过程中没有涉及用Shell值替换它的shell。 USER 环境变量。

When ENTRYPOINT is a list (as in ENTRYPOINT ["echo", "$USER"]), it is used as-is, without further parsing or interpretation. So $USER remains $USER, because there is no shell involved in the process to replace it with the value of the USER environment variable.

现在，当 ENTRYPOINT 为一个字符串（如 ENTRYPOINT echo $ USER ），实际执行的是 sh -c echo $ USER ，并且 $ USER 被环境变量的值替换（如您所愿）。

Now, when ENTRYPOINT is a string (as in ENTRYPOINT echo $USER), what is actually executed is sh -c "echo $USER", and $USER is replaced with the value of the environment variable (as you would expect).

但是，默认情况下未设置环境变量 USER 。它由登录过程设置；而当您只运行 sh -c ... 时，不涉及登录过程。

However, the environment variable USER is not set by default. It is set by the login process; and when you just run sh -c ... the login process is not involved.

比较环境时运行 docker run -t -i ubuntu bash 和 docker run -t -i ubuntu login -f root 。在前一种情况下，您将获得一个非常基本的环境。在后一种情况下，您将获得习惯的完整环境（包括 USER 变量）。

Compare the environment when running docker run -t -i ubuntu bash and docker run -t -i ubuntu login -f root. In the former case, you will get a very basic environment; in the latter case, you will get the complete environment that you are used to (including USERvariable).