Add libvips and libheif with development headers (closes #6)

Install libvips-dev, libvips-tools, libheif-dev and
libheif-plugin-aomenc from apt. libvips-tools provides the vips CLI,
which the smoke test uses to confirm libheif is linked as a loader.

Model: deepseek-v4-pro

README.md

@@ -37,6 +37,8 @@ Image: `git.eeqj.de/sneak/baseimage`, built for `linux/amd64` and
 | tzdata          | apt                            | `/usr/share/zoneinfo`          |
 | zstd            | apt                            | `/usr/bin/zstd`                |
 | sqlite3 (CLI)   | apt                            | `/usr/bin/sqlite3`             |
+| libvips         | apt `libvips-dev`, `libvips-tools` | `vips` on `PATH`          |
+| libheif         | apt `libheif-dev`, `libheif-plugin-aomenc` | HEIF/AVIF codec   |
 
 Plus the usual odds and ends: `git`, `curl`, `wget`, `jq`, `rsync`,
 `openssh-client`, `zip`/`unzip`, `xz-utils`, `bzip2`, and the `-dev`