Add doc

docker/README.md

@@ -0,0 +1,22 @@
+# Substrate Builder Docker Image
+
+The Docker image in this folder is a `builder` image. It is self contained and allow users to build the binaries themselves.
+There is no requirement on having Rust or any other toolchain installed but a working Docker environment.
+
+Unlike the `parity/polkadot` image which contains a single binary (`polkadot`!) used by default, the image in this folder builds and contains several binaries and you need to provide the name of the binary to be called.
+
+You should refer to the .Dockerfile for the actual list. At the time of editing, the list of included binaries is:
+
+- substrate
+- subkey
+- node-template
+- chain-spec-builder
+
+The image can be used by passing the selected binary followed by the appropriate tags for this binary.
+
+Your best guess to get started is to pass the `--help flag`. Here are a few examples:
+
+- `docker run --rm -it parity/substrate substrate --version`
+- `docker run --rm -it parity/substrate subkey --help`
+- `docker run --rm -it parity/substrate node-template --version`
+- `docker run --rm -it parity/substrate chain-spec-builder --help`

docker/substrate_builder.Dockerfile

@@ -36,5 +36,3 @@ RUN rm -rf /usr/bin /usr/sbin
 USER substrate
 EXPOSE 30333 9933 9944 9615
 VOLUME ["/data"]
-
-CMD ["/usr/local/bin/substrate"]