feat(RELEASE): allow overriding build.sh

* add `ARG` for `RELEASE_URL`
* make `build.sh` prepare `.tags` even if tag exists
* add outputs to be used by github workflows

Dockerfile

@@ -7,11 +7,12 @@ RUN  apk update \
     && update-ca-certificates    
 
 # Download TileBoard
-RUN wget -q -O release.zip "%RELEASE_URL%" \
+ARG RELEASE_URL
+RUN wget -q -O release.zip "${RELEASE_URL:-%RELEASE_URL%}" \
     && unzip release.zip -d /tileboard/ \
     && rm release.zip
 
 # Start Server
 WORKDIR /tileboard
 EXPOSE  8000
-ENTRYPOINT python3 -m http.server
+ENTRYPOINT python3 -m http.server