Add .gitattributes to enforce LF line endings

Shell/Python/C sources and the Dockerfile must stay LF so the Linux
build and the action entrypoint shebang work.

.gitattributes

@@ -0,0 +1,10 @@
+# Keep LF on everything the Linux build/runtime executes — CRLF breaks the
+# shebang in shell scripts and is undesirable in the C/Python/Docker sources.
+* text=auto eol=lf
+*.sh text eol=lf
+*.py text eol=lf
+*.c  text eol=lf
+Dockerfile text eol=lf
+*.yml text eol=lf
+*.yaml text eol=lf
+*.mpq binary