Clarify local binary name vs Dockerfile binary name

go build ./... names the binary after the module (gitea-heatmap-sidecar)
while the Dockerfile uses -o /heatmap. Fix .gitignore to match the local
name and add notes in both files explaining the discrepancy.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -125,6 +125,10 @@ Set `OP_TYPES=5,6,7,9,11,18,24` for a more inclusive count.
 go run . # requires DATABASE_URL and ALLOWED_USERS in env
 ```
 
+`go build ./...` produces a binary named `gitea-heatmap-sidecar` (derived from
+the module name). The Dockerfile overrides this with `-o /heatmap`; the two
+names differ but refer to the same program.
+
 On NixOS:
 
 ```bash