Run xmrblocks with exec
**TL;DR**:
The following error is thrown when doing:
```bash
exec xmrblocks -b /monero
```

```bash
2018-10-12 04:36:04,010 INFO  [default] Page size: 4096
"/monero"
2018-10-12 04:36:05.016	    7f79ba675780	WARN 	blockchain.db.lmdb	src/blockchain_db/lmdb/db_lmdb.cpp:75	Failed to create a read transaction for the db: Resource temporarily unavailable
terminate called after throwing an instance of 'cryptonote::DB_ERROR_TXN_START'
  what():  Failed to create a read transaction for the db: Resource temporarily unavailable
```

---

I can run `xmrblocks`, it's working fine.
```bash
xmrblocks -b /monero
```

I would like to run it within a docker container.
However, for this purpose I use an entrypoint script like this:
```bash
#!/bin/bash


# used for xmrblocks
OPTIONS="-b $LMDB_PATH"  #  --port $PORT  --enable-autorefresh-option=$ENABLE_AUTOREFRESH"

if [[ "${1:0:1}" = '-' ]]  || [[ -z "$@" ]]; then
  set -- "xmrblocks $@ $OPTIONS"
fi

# allow the container to be started with `--user
if [ "$(id -u)" = 0 ]; then
  adduser --system --group --uid "$USER_ID" --shell /bin/false xmrblocks &> /dev/null
  exec su-exec xmrblocks xmrblocks $@
fi

exec $@
```

These are the lines that cause trouble:
```bash
  exec su-exec xmrblocks xmrblocks $@
  exec $@
```

If I leave the `exec`, everything will work just fine.

With the `exec`, the following error appears and `xmrblocks` just stops.
```bash
2018-10-12 04:36:04,010 INFO  [default] Page size: 4096
"/monero"
2018-10-12 04:36:05.016	    7f79ba675780	WARN 	blockchain.db.lmdb	src/blockchain_db/lmdb/db_lmdb.cpp:75	Failed to create a read transaction for the db: Resource temporarily unavailable
terminate called after throwing an instance of 'cryptonote::DB_ERROR_TXN_START'
  what():  Failed to create a read transaction for the db: Resource temporarily unavailable
```

Is there something I am missing?
