a full access policy, designed to assign a subdirectory/subkey to a policy. Think s3://bucket/environmet/dev, only assigning dev to this policy.

full-access-s3-subkey-policy.json

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": [
        "s3:ListAllMyBuckets",
        "s3:GetBucketLocation"
      ],
      "Effect": "Allow",
      "Resource": [
        "arn:aws:s3:::*"
      ]
    },
    {
      "Effect": "Allow",
      "Action": "s3:ListBucket",
      "Resource": "arn:aws:s3:::{bucket}",
      "Condition": {
        "StringLike": {
          "s3:prefix": [
            "",
            "{key}",
            "{key}/*",
            "{key}/{subkey}",
            "{key}/{subkey}/*"
          ]
        }
      }
    },
    {
      "Action": [
        "s3:*"
      ],
      "Effect": "Allow",
      "Resource": [
        "arn:aws:s3:::{bucket}/{key}/{subkey}",
        "arn:aws:s3:::{bucket}/{key}/{subkey}/*"
      ]
    }
  ]
}