readme: update 'build manually' instructions

Public build assumes clang-12. Also adds a reference to instructions for
using the pre-baked docker image with the frozen toolchain

Signed-off-by: Ivo Jimenez <[email protected]>

README.md

@@ -74,10 +74,10 @@ all built from source. The only thing we do not build yet is the Linux Kernel, b
 For hackers, here is the short and sweet:
 
 ```
-sudo ./install-dependencies.sh && CC=gcc-11 CXX=g++-11 ./build.sh
+sudo ./install-dependencies.sh && CC=clang CXX=clang++ ./build.sh
 ```
 
-> We build with coros, so you need gcc11+ or clang12+
+For quicker dev setup, we provide a [docker image](tools/docker/README.md) with the toolchain installed.
 
 # Beta builds
 

tools/docker/README.md

@@ -0,0 +1,25 @@
+# Docker Image with Redpanda Toolchain
+
+The `Dockerfile` available in this folder defines the contents of the image published to <https://hub.docker.com/r/vectorized/redpanda-toolchain>. 
+To build Redpanda using this image:
+
+```bash
+docker pull vectorized/redpanda-toolchain
+
+cd redpanda/
+docker run --rm -ti -v $PWD:$PWD:Z -w $PWD vectorized/redpanda-toolchain ./build.sh
+```
+
+You can add the `--ipc=host` flag to share the host's `/dev/shm` folder, so that the `CCACHE_DIR` folder (`/dev/shm/redpanda` by default) can be available across `docker run` invocations. 
+
+In systems with SELinux enabled, the `:Z` bind-mount option might cause the container to take too long to setup (due to SELinux labels being applied to individual files). 
+To avoid this, you can add the `--privileged` flag, and that will instruct Docker (or Podman) to skip applying SELinux profiles. 
+
+If you need rebuild the toolchain image locally:
+
+```bash
+docker build \
+ -t vectorized/redpanda-toolchain \
+ -f tools/docker/Dockerfile \
+ .
+```