Add support for adding CMD to images

Previously we were inconsistent here and were adding CMD for kiwi images and
ENTRYPOINT for docker images. Now you can choose which one to use.

src/bci_build/package.py

@@ -141,6 +141,9 @@ class BaseContainerImage(abc.ABC):
     #: An optional entrypoint for the image, it is omitted if empty or ``None``
     entrypoint: Optional[str] = None
 
+    #: An optional CMD for the image, it is omitted if empty or ``None``
+    cmd: Optional[str] = None
+
     #: Extra environment variables to be set in the container
     env: Union[Dict[str, Union[str, int]], Dict[str, str], Dict[str, int]] = field(
         default_factory=dict

src/bci_build/templates.py

@@ -33,6 +33,7 @@
 
 {{ image.env_lines }}
 {% if image.entrypoint -%}ENTRYPOINT {{ image.entrypoint }}{% endif %}
+{% if image.cmd -%}CMD {{ image.cmd }}{% endif %}
 {{ image.dockerfile_custom_end }}
 """
 )
@@ -76,7 +77,8 @@
 {{ image.extra_label_xml_lines }}
           </suse_label_helper:add_prefix>
         </labels>
-{% if image.entrypoint %}        <subcommand execute="{{ image.entrypoint }}"/>{% endif %}
+{% if image.cmd %}        <subcommand execute="{{ image.cmd }}"/>{% endif %}
+{% if image.entrypoint %}        <entrypoint execute="{{ image.entrypoint }}"/>{% endif %}
 {{ image.kiwi_env_entry }}
       </containerconfig>
     </type>