Add container readiness checks and exclude local state from images

Adapted from MikaelKW/nodecast-tv-plus readiness and container context work.

Co-authored-by: MikaelKW <87399352+MikaelKW@users.noreply.github.com>

.dockerignore

@@ -0,0 +1,19 @@
+# Keep local credentials, state, and build output out of the image.
+.git
+.github
+.codex
+.agents
+.aws
+.env*
+node_modules
+data
+transcode-cache
+coverage
+dist
+release
+test
+tests
+test-results
+playwright-report
+*.log
+*.patch