ARM version of json-server

[francescor]

we needed the ARM image of the Docker image vimagick/json-server https://hub.docker.com/r/vimagick/json-server
as in #211 (comment)

In the meanwhile we've forked it in
https://gitlab.com/opencity-labs/vimagick-dockerfiles-json-server

where ARM version is available at https://gitlab.com/opencity-labs/vimagick-dockerfiles-json-server/container_registry/8084209

btw: this image registry.gitlab.com/opencity-labs/vimagick-dockerfiles-json-server:json-server-0.17.4-arm has been build with the latest v.0.* of the json-server, which is the latest that support --middleware