Enable Github actions build of Docker image

[sethforprivacy]

Hi there,

This PR is very much a "use if you want, no offense if you don't" PR, but enables Docker image builds automatically:

• Weekly w/o caching to ensure all base images/packages are up to date to remediate vulns via regular updates
• When any commit is pushed, unless the commit only edits README.md

The Docker image is built and pushed to Github Packages and Docker Hub with the latest tag and the MONERO_BRANCH arg as a tag as well.

Note that for this to push to Docker Hub, you will need to create two repository secrets:

• DOCKER_USERNAME - your Docker Hub username
• DOCKER_PASSWORD - your Docker Hub password or access token

If you don't want to push to Docker Hub, I can update and remove all relevant Docker Hub configuration.

And if you don't want to build and push images at all, feel free to simply decline the PR :) The main advantage of this is users can utilize the project without needing to bother with manual builds, git pull's, etc., and I will be using it to start bundling the explorer with monerod in a few of my projects/guides.

You can see working runs of this set of actions here:

https://github.com/sethforprivacy/onion-monero-blockchain-explorer/actions

And Docker images built using it are available here:

https://hub.docker.com/repository/docker/sethsimmons/xmrblocks

[moneroexamples]

Thanks. I will have a look at dockerhub.

[moneroexamples]

@sethforprivacy

Sorry it took so long, but finally made an account on dockerhub and created the two secrets. I re-based your PR to dockerhub branch on the explorer and will merge it there for test. Not sure if changing the branch will affect the actions.