containertool should select an appropriate default base image for static and dynamic executables

`containertool` reads the architecture from the executable's ELF header and uses it to choose the correct base image manifest from a multi-arch base image (e.g. it will choose an aarch64 base image for an aarch64 executable).

It could be helpful to expand this to choose different default base images for statically- and dynamically-linked executables. The ELF reader would need to be improved to read the necessary information.

* if the executable is statically linked, use `DEFAULT_STATIC_BASE_IMAGE`, which could be a distroless image.
* if the executable is dynamically linked, use `DEFAULT_DYNAMIC_BASE_IMAGE`, which could be the current default of `swift:slim`

Going further, dynamic executables linked with `--static-swift-stdlib` could use a third default base image which provides the usual Linux dynamic libraries but does not include the Swift runtime, as this is not necessary.

This feature could be guarded behind a `--automatic-base-image` flag.

The issue names no files or tests. Start by locating containertool's ELF reader and current default base-image selection, then trace how architecture and linkage are determined. Done means static and dynamic executables select the requested default images, with the automatic-base-image flag behavior and these cases covered by tests.

Written by the indexing model from the issue text.