Tools that create 'encrypted file containers' use open encryption standards like AES to encrypt container contents, however the container format is proprietary, I am guessing, so need the same tool to decrypt the data. So, is there any Open 'Encrypted File Container format'? I mean I should be able to use one tool to create the container, maybe another to mount/decrypt the container if they both support the same standard?
Archives and emails come to mind.
Email formats: S/MIME, aka Cryptographic Message Syntax, aka PKCS#7 is an open standard for a container format that supports cryptographic signing and encryption (both symmetric encryption like AES, and public-key encryption like RSA). S/MIME is widely supported by email clients such as Outlook, Thunderbird, and probably many others.
OpenPGP Message Format is another such container format designed for email which supports the same crypto operations as S/MIME. In addition, many people use PGP for encrypting files on disk. In fact all software distributed through a linux package manager uses the PGP format for its crypto wrapper. The most common tool for reading / writing PGP files is called GnuPG aka GPG, but others tools exist as well.
TL;DR: Yes, there are many such Open 'Encrypted File Container formats' which can be understood by many different tools. The ones that come to mind are zip-like things, and email-like things, but I'm sure there are more examples.