test: add container test suites and run them in CI (#111)

Adds structure, compat and feature-gated container test suites (test/run.sh), runs them in CI, and gates image publishing on a green test run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitattributes

@@ -0,0 +1,8 @@
+# Shell scripts and the Dockerfile are copied into a Linux image. With CRLF
+# line endings the shebang resolves to "/bin/sh\r" and the container fails to
+# start, which happens on Windows checkouts with core.autocrlf=true. Pin them
+# to LF so the image builds the same way on every platform.
+*.sh text eol=lf
+Dockerfile text eol=lf
+*.yml text eol=lf
+*.txt text eol=lf