Add option to specify build and runtime environment variables

[fridex]

As an Amun user, I would like to have an ability to set environment variables that get injected into container on build time as well as environment variables that get injected on runtime. For this purpose we should introduce a separate configuration option in the JSON supplied on endpoint:

{
  "base": "fedora:28",
  "build": {
     "env": [
      {"name": "FOO", "value": "BAR"}
    ]
  },
  "run": {
     "env": [
      {"name": "BAZ", "value": "FOZ"}
    ]
  }
}