command_executer: Add documentation and Dockerfile (#20)

This helper tool was missing documentation for how to use it, which
in my opinion, diminished the great value it added when testing
enclaves, as without it you wouldn't have an elegant way of sending
shell commands in the enclave.

Contrary to what others might think, just reading the help of the
program isn't enough.

Signed-off-by: Sabin Rapan <[email protected]>

samples/command_executer/README.md

@@ -0,0 +1,69 @@
+# command-executer
+
+An application that can run either as a server or a client. When running as a
+server, it listens on a port for commands and executes them afterwards, sending
+the output back to the client. When running as a client, it connects to the
+above mentioned port, sends commands and waits for the reply (unless --no-wait
+is used). The command-executer is useful for executing shell commands inside
+the enclave and sending/receiving files to/from the enclave.
+
+It replaces the old nc-vsock we used in the past (which we haven't bothered to
+remove yet).
+
+## Building
+
+```
+	$ cargo build
+```
+
+## Running
+
+1. Build the project (see above).
+
+2. Use the Dockerfile in resources/ either as an example or as is
+and build an EIF.
+
+```
+	$ export NITRO_CLI_BLOBS=$(realpath ../../blobs/)
+	$ nitro-cli build-enclave --docker-dir "./resources" --docker-uri mytag --output-file command-executer.eif
+```
+
+_NOTE: these steps can either be done on your local machine or on the EC2
+instance your going to launch the enclave._
+
+3. Copy __both__ the EIF __and__ the command-executer binary to the EC2
+instance you are about to run an enclave on.
+
+4. Launch an enclave with the EIF containing command-executer.
+
+```
+	$ ./nitro-cli run-enclave --cpu-count 4 --memory 2048 --eif-path command_executer.eif
+	Start allocating memory...
+	Running on instance CPUs [1, 5, 2, 6]
+	Started enclave with enclave-cid: 16, memory: 2048 MiB, cpu-ids: [1, 5, 2, 6]
+	Sending image to cid: 16 port: 7000
+	{
+	  "EnclaveID": "i-08aa8a2f7bff2ff99_enc103923520469154997",
+	  "EnclaveCID": 16,
+	  "NumberOfCPUs": 4,
+	  "CPUIDs": [
+	    1,
+	    5,
+	    2,
+	    6
+	  ],
+	  "MemoryMiB": 2048
+	}
+```
+
+5. Use the command-executer to send shell commands to the enclave
+
+```
+	$ ./command-executer run --cid 16 --port 5005 --command "whoami"
+```
+
+6. Use the command-executer to send files to the enclave (e.g. binaries you built in the instance)
+
+```
+	$ ./command-executer send-file --cid 16 --localpath "./stress-ng" --port 5005 --remotepath "/usr/bin/stress-ng"
+```

samples/command_executer/resources/Dockerfile

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+FROM ubuntu:latest
+  COPY command-executer .
+  RUN apt-get update && apt-get install -y \
+      apt-utils
+  CMD ./command-executer listen --port 5005