build: make build-and-push.sh runnable from any directory

- Resolve PROJECT_DIR from the script's own location by default
  (works with absolute, relative and symlinked invocations)
- Allow override via --project-dir flag or PROJECT_DIR env var
- Improve error message for invalid --project-dir paths
- Update usage and README accordingly

README.md

@@ -18,6 +18,10 @@ The script reads `artifactId` and `version` from `pom.xml` for the image name
 and tag, builds for `linux/amd64`, and verifies the image is present on the
 remote after pushing. See `./scripts/build-and-push.sh --help` for all flags.
 
+It can be invoked from any directory: by default it resolves the project root
+from the script's own location. Override with `--project-dir PATH` or the
+`PROJECT_DIR` environment variable when needed.
+
 ## Running the application in dev mode
 
 You can run your application in dev mode that enables live coding using: