Add Docker multi-platform build workflow

[alexcos20]

GitHub Action Event	Example Input	Resulting Docker Tag(s)
Push to main branch	git push origin main	main
Push to any other branch	git push origin dev	none, no automated build
Pull Request	PR #42	pr-42
Push a semver tag	git tag v1.2.3	1.2.3 + latest

Build container image and pushes it to both dockerhub & ghcr

#docker inspect oceanprotocol/df-py:pr-843


Name:      docker.io/oceanprotocol/df-py:pr-843
MediaType: application/vnd.oci.image.index.v1+json
Digest:    sha256:7a9efa17c04b9bea2a9f894ab1d26e3624542a8aa24142a96997c11b38be504b
           
Manifests: 
  Name:        docker.io/oceanprotocol/df-py:pr-843@sha256:19bbd43c8118e3ec3bdec208684ae12c92b85c6e4cd8df913d1447cc738ae742
  MediaType:   application/vnd.oci.image.manifest.v1+json
  Platform:    linux/amd64
               
  Name:        docker.io/oceanprotocol/df-py:pr-843@sha256:b4c4a92f84019609570d29e2680fe7ff0128f187c9c3090fb7bf5b15a8e55f2a
  MediaType:   application/vnd.oci.image.manifest.v1+json
  Platform:    unknown/unknown
  Annotations: 
    vnd.docker.reference.digest: sha256:19bbd43c8118e3ec3bdec208684ae12c92b85c6e4cd8df913d1447cc738ae742
    vnd.docker.reference.type:   attestation-manifest

#docker inspect ghcr.io/oceanprotocol/df-py:pr-843
Name:      ghcr.io/oceanprotocol/df-py:pr-843
MediaType: application/vnd.oci.image.index.v1+json
Digest:    sha256:26c812e54e29ab64dbdcc8b4ef5357099a2d97b9cad03e58411f6bedd2de053b
           
Manifests: 
  Name:        ghcr.io/oceanprotocol/df-py:pr-843@sha256:19bbd43c8118e3ec3bdec208684ae12c92b85c6e4cd8df913d1447cc738ae742
  MediaType:   application/vnd.oci.image.manifest.v1+json
  Platform:    linux/amd64
               
  Name:        ghcr.io/oceanprotocol/df-py:pr-843@sha256:b2433d6b5591ffbbdb810dad237ade6d43e1f81fad167b27de3340c6bdcf3a0f
  MediaType:   application/vnd.oci.image.manifest.v1+json
  Platform:    unknown/unknown
  Annotations: 
    vnd.docker.reference.digest: sha256:19bbd43c8118e3ec3bdec208684ae12c92b85c6e4cd8df913d1447cc738ae742
    vnd.docker.reference.type:   attestation-manifest
    ```