Change Container environment variables to use DOTNET_ prefix

[baronfel]

Other areas of the SDK and Runtime use the DOTNET_ prefix to prevent overlapping. We should do the same, but we should keep compatibility with the old environment variable names for at least one release.

We should use DOTNET_CONTAINER as a prefix where applicable.

          i see that it's an existing pattern, but DOTNET_CONTAINER_ or DOTNET_SDK_CONTAINER_ would be a better namespace prefix.

Originally posted by @kasperk81 in #41506 (comment)

[kasperk81]

dup #41609

[baronfel]

Oh, perfect. Thanks @Kasper81!