Update README and Dockerfile for new dependency on arm64

[FalcoXYZ]

Changes:

Updated README to add the symlink for arm64 architecture.
Updated Dockerfile to add the libgflags-dev dependency for arm64 architecture.

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Testing

Requires testing

• Yes
• No

In case you checked yes, did you write tests??

• Yes
• No

Tested Dockerfile on arm64 EC2, libgflags-dev was installed as expected.