Add .dockerignore to shrink the build context sent to the daemon

node_modules/.git/dist were being sent on every build with no
.dockerignore at all — same class of fix as goonk's recent deploy-speed
pass. Verified by building with the new file in place (adventure and
new spot-checked with a full container run + health check; the rest
share the same multi-stage Dockerfile pattern).

.dockerignore

@@ -0,0 +1,9 @@
+node_modules
+**/node_modules
+dist
+**/dist
+.git
+.env
+.env.example
+*.log
+data