When base image layers are cached locally or referenced during image
composition, they may be stored with generic names that don't reflect
their actual compression format (e.g., 'layer.tar' for a gzip file,
or no extension at all).

The previous extension-based detection would incorrectly classify
these as uncompressed layers, leading to manifest media type mismatches.
When pushing with OCI format, this caused layers to be declared as
application/vnd.oci.image.layer.v1.tar (uncompressed) when they were
actually gzip-compressed, resulting in 'invalid tar header' errors
during docker pull.

This change modifies detect_compression_type() to:
1. First check file magic bytes (1f 8b for gzip, 28 b5 2f fd for zstd)
2. Fall back to extension-based detection if magic bytes don't match

This ensures correct media type detection regardless of filename.

Added comprehensive unit tests covering:
- Correct detection via magic bytes
- Wrong extension scenarios (the bug case)
- No extension scenarios
- Fallback to extension-based detection still works

Fixes issue where layer blobs were gzip-compressed but declared as
uncompressed in OCI manifests, causing docker pull failures.

See: https://github.com/bazeltools/rules_minidock_tools/issues/XXX