[Docs] Improve Kubernetes documentation

Minor updates, incl. the description of `Default image`, and `privileged` for NCCL tests

Author: peterschmidt85

docs/docs/concepts/dev-environments.md

@@ -133,7 +133,7 @@ The `gpu` property lets you specify vendor, model, memory, and count — e.g., `
 
 If vendor is omitted, `dstack` infers it from the model or defaults to `nvidia`.
 
-??? info "Google Cloud TPU"
+<!-- ??? info "Google Cloud TPU"
     To use TPUs, specify its architecture via the `gpu` property.
 
     ```yaml
@@ -146,7 +146,7 @@ If vendor is omitted, `dstack` infers it from the model or defaults to `nvidia`.
       gpu: v2-8
     ```
 
-    Currently, only 8 TPU cores can be specified, supporting single TPU device workloads. Multi-TPU support is coming soon.
+    Currently, only 8 TPU cores can be specified, supporting single TPU device workloads. Multi-TPU support is coming soon. -->
 
 ??? info "Shared memory"
     If you are using parallel communicating processes (e.g., dataloaders in PyTorch), you may need to configure 
@@ -159,8 +159,8 @@ If vendor is omitted, `dstack` infers it from the model or defaults to `nvidia`.
 
 #### Default image
 
-If you don't specify `image`, `dstack` uses its base Docker image pre-configured with 
-`uv`, `python`, `pip`, essential CUDA drivers, and NCCL tests (under `/opt/nccl-tests/build`). 
+If you don't specify `image`, `dstack` uses its [base :material-arrow-top-right-thin:{ .external }](https://github.com/dstackai/dstack/tree/master/docker/base){:target="_blank"} Docker image pre-configured with 
+    `uv`, `python`, `pip`, essential CUDA drivers, `mpirun`, and NCCL tests (under `/opt/nccl-tests/build`). 
 
 Set the `python` property to pre-install a specific version of Python.
 

docs/docs/concepts/services.md

@@ -433,8 +433,8 @@ If vendor is omitted, `dstack` infers it from the model or defaults to `nvidia`.
 
 #### Default image
 
-If you don't specify `image`, `dstack` uses its base Docker image pre-configured with 
-`uv`, `python`, `pip`, essential CUDA drivers, and NCCL tests (under `/opt/nccl-tests/build`). 
+If you don't specify `image`, `dstack` uses its [base :material-arrow-top-right-thin:{ .external }](https://github.com/dstackai/dstack/tree/master/docker/base){:target="_blank"} Docker image pre-configured with 
+    `uv`, `python`, `pip`, essential CUDA drivers, `mpirun`, and NCCL tests (under `/opt/nccl-tests/build`). 
 
 Set the `python` property to pre-install a specific version of Python.
 

docs/docs/concepts/tasks.md

@@ -229,8 +229,6 @@ If vendor is omitted, `dstack` infers it from the model or defaults to `nvidia`.
 <!-- ??? info "Google Cloud TPU"
     To use TPUs, specify its architecture via the `gpu` property.
 
-    <!-- TODO: Add a TRL TPU example -->
-
     ```yaml
     type: task
     name: train    
@@ -259,8 +257,8 @@ If vendor is omitted, `dstack` infers it from the model or defaults to `nvidia`.
 
 #### Default image
 
-If you don't specify `image`, `dstack` uses its base Docker image pre-configured with 
-`uv`, `python`, `pip`, essential CUDA drivers, and NCCL tests (under `/opt/nccl-tests/build`). 
+If you don't specify `image`, `dstack` uses its [base :material-arrow-top-right-thin:{ .external }](https://github.com/dstackai/dstack/tree/master/docker/base){:target="_blank"} Docker image pre-configured with 
+    `uv`, `python`, `pip`, essential CUDA drivers, `mpirun`, and NCCL tests (under `/opt/nccl-tests/build`). 
 
 Set the `python` property to pre-install a specific version of Python.
 

examples/clusters/nccl-tests/.dstack.yml

@@ -21,6 +21,9 @@ commands:
       sleep infinity
     fi
 
+# Uncomment if the `kubernetes` backend requires it for `/dev/infiniband` access
+#privileged: true
+
 resources:
   gpu: nvidia:1..8
   shm_size: 16GB

examples/clusters/nccl-tests/README.md

@@ -33,21 +33,22 @@ commands:
       sleep infinity
     fi
 
+# Uncomment if the `kubernetes` backend requires it for `/dev/infiniband` access
+#privileged: true
+
 resources:
   gpu: nvidia:1..8
   shm_size: 16GB
 ```
 
 </div>
 
-<!-- TODO: Need to stop using our EFA image - either make our default image cluster-friendly, or recommend using NGC or other images -->
+!!! info "Default image"
+    If you don't specify `image`, `dstack` uses its [base :material-arrow-top-right-thin:{ .external }](https://github.com/dstackai/dstack/tree/master/docker/base){:target="_blank"} Docker image pre-configured with 
+    `uv`, `python`, `pip`, essential CUDA drivers, `mpirun`, and NCCL tests (under `/opt/nccl-tests/build`). 
 
-!!! info "Docker image"
-    The `dstackai/efa` image used in the example comes with MPI and NCCL tests pre-installed. While it is optimized for
-    [AWS EFA :material-arrow-top-right-thin:{ .external }](https://aws.amazon.com/hpc/efa/){:target="_blank"}, it can also
-    be used with regular TCP/IP network adapters and InfiniBand. 
-    
-    See the [source code :material-arrow-top-right-thin:{ .external }](https://github.com/dstackai/dstack/blob/master/docker/efa) for the image.
+!!! info "Privileged"
+    In some cases, the backend (e.g., `kubernetes`) may require `privileged: true` to access the high-speed interconnect (e.g., InfiniBand).
 
 ### Apply a configuration