containertool: Add basic ELF file type detection

[euanh]

Motivation

The container's architecture metadata field should match the architecture of the executable it contains. The plugin can't currently tell which architecture the SDK targets, but containertool can infer the architecture by reading the ELF headers.

Modifications

• Adds basic ELF header reader. Reading the whole ELF header is not required - the necessary information is provided by the first few fields of the header.
• Changes containertool to use the detected architecture unless overridden by command line flags or environment variables.

Result

Adding ELF detection reduces the risk of building a mismatched container image, where the architecture of the packaged binary does not match the architecture of the underlying Linux distribution.

Test Plan

• New unit tests exercise ELF header parsing
• A new integration test checks that containertool selects the correct base image architecture for different cross-compiled binaries
• All previous tests continue to pass

Fixes #49