Fixed minio image upload InvalidBucketNameError error

[abola]

Add new attribute codimd.imageUpload.minio.bucket link to secret environment CMD_S3_BUCKET

Before this fix, must define bucket name in s3 attributes and with one fake accessKeyId like below.

codimd:
  (....)
  imageUpload:
    storeType: minio
    minio:
      endpoint: minio-host
      secure: false
      port: 9000
      accessKey: abcd1234
      secretKey: abcd1234
    s3:
      accessKeyId: fake_id_here
      bucket: codimd

This PR add bucket to minio attributes to fix this prob

codimd:
  (....)
  imageUpload:
    storeType: minio
    minio:
      endpoint: minio_host
      secure: false
      port: 9000
      accessKey: abcd1234
      secretKey: abcd1234
      bucket: codimd