Build rocker/r-ver:devel for linux/arm64 platform

[eitsupi]

The request was made in #144 (comment)

This should be achieved simply by adding the platforms array to the stack file and automatically generating the bakefile.

      "platforms": [
        "linux/amd64",
        "linux/arm64"
      ]

rocker-versioned2/stacks/devel.json

Lines 20 to 41 in d9dcc2c

	{
	"IMAGE": "r-ver",
	"labels": {
	"org.opencontainers.image.title": "rocker/r-ver",
	"org.opencontainers.image.description": "Reproducible builds to fixed version of R"
	},
	"FROM": "ubuntu:latest",
	"ENV": {
	"R_VERSION": "devel",
	"R_HOME": "/usr/local/lib/R",
	"TZ": "Etc/UTC"
	},
	"COPY_a_script": "scripts/install_R_source.sh /rocker_scripts/install_R_source.sh",
	"RUN_a_script": "/rocker_scripts/install_R_source.sh",
	"ENV_after_a_script": {
	"CRAN": "https://cloud.r-project.org",
	"LANG": "en_US.UTF-8"
	},
	"COPY": "scripts /rocker_scripts",
	"RUN": "/rocker_scripts/setup_R.sh",
	"CMD": "[\"R\"]"
	},