Add ImageMagick and Ghostscript (closes #7)

Install imagemagick (ImageMagick 7, so the CLI is magick) and
ghostscript from apt and check both in the smoke test.

Model: deepseek-v4-pro

README.md

@@ -39,6 +39,8 @@ Image: `git.eeqj.de/sneak/baseimage`, built for `linux/amd64` and
 | 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   |
+| ImageMagick     | apt                            | `magick`                       |
+| Ghostscript     | apt                            | `gs`                           |
 
 Plus the usual odds and ends: `git`, `curl`, `wget`, `jq`, `rsync`,
 `openssh-client`, `zip`/`unzip`, `xz-utils`, `bzip2`, and the `-dev`

bin/test

@@ -28,6 +28,8 @@ docker run --rm "$img" bash -ec '
     sqlite3 --version
     vips --version
     vips -l | grep -i heif > /dev/null
+    magick --version
+    gs --version
 '
 
 echo "== $img: runit service lifecycle"

build.sh

@@ -29,7 +29,8 @@ apt-get -y install --no-install-recommends \
     shellcheck \
     tzdata \
     zstd sqlite3 \
-    libvips-dev libvips-tools libheif-dev libheif-plugin-aomenc
+    libvips-dev libvips-tools libheif-dev libheif-plugin-aomenc \
+    imagemagick ghostscript
 
 # --- Go (latest) -------------------------------------------------------------
 gover="$(curl -fsSL 'https://go.dev/VERSION?m=text' | head -n1)"
@@ -110,6 +111,8 @@ zstd --version
 sqlite3 --version
 vips --version
 vips -l | grep -i heif > /dev/null
+magick --version
+gs --version
 hugo version
 node --version
 npm --version