Added note about custom base images (#14125)

Co-authored-by: awaelchli <aedu.waelchli@gmail.com>

docs/source-app/core_api/lightning_work/compute_content.rst

@@ -22,6 +22,15 @@ a :class:`~lightning_app.utilities.packaging.cloud_compute.CloudCompute` to your
     # Run on a fast multi-GPU machine (see specs below)
     MyCustomWork(cloud_compute=L.CloudCompute("gpu-fast-multi"))
 
+.. warning::
+     Custom base images are not supported with the default CPU cloud compute. For example:
+
+     .. code-block:: py
+
+         class MyWork(LightningWork):
+             def __init__(self):
+              super().__init__(cloud_build_config=BuildConfig(image="my-custom-image")) # no cloud compute, for example default work
+
 
 Here is the full list of supported machine names:
 

docs/source-app/glossary/build_config/build_config_basic.rst

@@ -35,7 +35,6 @@ When the LightningWork starts up, it will pick up the requirements file if prese
 .. note::
     This only applies when running in the cloud. The requirements.txt files get ignored when running locally.
 
-
 ----
 
 ***********************************
@@ -58,3 +57,12 @@ Instead of listing the requirements in a file, you can also pass them to the Lig
 
 .. note::
     The build config only applies when running in the cloud and gets ignored otherwise. A local build config is currently not supported.
+
+.. warning::
+     Custom base images are not supported with the default CPU cloud compute. For example:
+
+     .. code-block:: py
+
+         class MyWork(LightningWork):
+             def __init__(self):
+              super().__init__(cloud_build_config=BuildConfig(image="my-custom-image")) # no cloud compute, for example default work

docs/source-app/glossary/build_config/build_config_intermediate.rst

@@ -42,7 +42,15 @@ If you need to install additional system packages or run other configuration ste
                 # Can also be combined with extra requirements
                 self.cloud_build_config = CustomBuildConfig(requirements=["torchmetrics"])
 
-
 .. note::
     - When you need to execute commands or install tools that require more privileges than the current user has, you can use ``sudo`` without needing to provide a password, e.g., when installing system packages.
     - The build config only applies when running in the cloud and gets ignored otherwise. A local build config is currently not supported.
+
+.. warning::
+     Custom base images are not supported with the default CPU cloud compute. For example:
+
+     .. code-block:: py
+
+         class MyWork(LightningWork):
+             def __init__(self):
+              super().__init__(cloud_build_config=BuildConfig(image="my-custom-image")) # no cloud compute, for example default work