feat: refactored Dockerfile (#1375)

- Removed VIKUNJA_VERSION and custom git checkout, because it is not found in the repository. So it is not used anywhere.
- Optimized runner commands order
- Removed run.sh (it is not needed in fact)

Co-authored-by: Yurii Vlasov <yv@itsvit.org>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1375
Co-authored-by: Yurii Vlasov <yuriy@vlasov.pro>
Co-committed-by: Yurii Vlasov <yuriy@vlasov.pro>

docker/entrypoint.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env sh
+set -e
+
+if [ -n "$PUID" ] && [ "$PUID" -ne 0 ] && \
+   [ -n "$PGID" ] && [ "$PGID" -ne 0 ] ; then
+  echo "info: creating the new user vikunja with $PUID:$PGID"
+  addgroup -g "$PGID" vikunja
+  adduser -s /bin/sh -D -G vikunja -u "$PUID" vikunja -h /app/vikunja -H
+  chown -R vikunja:vikunja ./
+  su -pc /app/vikunja/vikunja - vikunja "$@"
+else
+  echo "info: creation of non-root user is skipped"
+  exec /app/vikunja/vikunja "$@"
+fi
+