sapphire-localnet: Add --no-explorer flag to disable Nexus and Explorer

[matevz]

Spinning up the web server and nexus indexer in the Localnet docker image causes additional delay and consumes CPU/memory/db cycles over longer run. For environments that don't need the indexer and the explorer (e.g. CI), add the --no-explorer flag to disable it.

TODO:

• implement --no-explorer flag
• benchmark startup times if they're reduced, see if you can speed it up more by shuffling start order
• also check, if there are any leftovers from indexer/explorer (are those stripped) so you can also reduce the image size
• besides updating Localnet docs in this repo, also update it here https://docs.oasis.io/dapp/tools/localnet
• I suggest we also change the explorer port from the standard 80 to 8548. In practice you will always map the port to sth else, so the Localnet Explorer available at http://localhost:80 message is incorrect. Also this reduces the number of -p docker parameters needed.