Ad docs on how to run regression tests

Reflects the changes in falcosecurity/falco#584.

content/docs/source.md

@@ -5,7 +5,7 @@ weight: 6
 
 Building falco requires having `cmake` and `g++` installed.
 
-## Build using docker-builder container
+## Build using falco-builder container
 
 One easy way to build falco is to run the [falco-builder](https://hub.docker.com/r/falcosecurity/falco-builder) container. It contains the reference toolchain we use to build packages.
 
@@ -20,15 +20,35 @@ built for the files in the centos image, not the host.
 * `BUILD_WARNINGS_AS_ERRORS`: consider all build warnings fatal
 * `MAKE_JOBS`: passed to the -j argument of make
 
-A typical way to run this builder is the following. Assumes you have
+A typical way to run this builder is the following. Assuming you have
 checked out falco and sysdig to directories below /home/user/src, and
-want to use a build directory of /home/user/build/falco:
+want to use a build directory of /home/user/build/falco, you would run
+the following:
 
 ```bash
 docker run --user $(id -u):$(id -g) -v /etc/passwd:/etc/passwd:ro -e MAKE_JOBS=4 -it -v /home/user/src:/source -v /home/user/build/falco:/build falcosecurity/falco-builder cmake
 docker run --user $(id -u):$(id -g) -v /etc/passwd:/etc/passwd:ro -e MAKE_JOBS=4 -it -v /home/user/src:/source -v /home/user/build/falco:/build falcosecurity/falco-builder package
 ```
 
+### Test using falco-tester container
+
+If you'd like to run the regression test suite against your build, you can use the [falco-tester](https://hub.docker.com/r/falcosecurity/falco-tester) container. Like the builder image, it contains the necessary environment to run the regression tests, but relies on a source directory and build directory that are mounted into the image. It's a different image than `falco-builder` as it doesn't need a compiler and needs a different base image to include the test runner framework [avocado](http://avocado-framework.github.io/).
+
+It does build a new container image `falcosecurity/falco:test` to test the process of buillding and running a container with the falco packages built during the build step.
+
+The image depends on the following parameters:
+
+* `FALCO_VERSION`: The version of the falco package to include in the test container image.
+
+A typical way to run this builder is the following. Assuming you have
+checked out falco and sysdig to directories below /home/user/src, and
+want to use a build directory of /home/user/build/falco, you would run
+the following:
+
+```bash
+docker run --user $(id -u):$(id -g) -v /boot:/boot:ro -v /var/run/docker.sock:/var/run/docker.sock -v /etc/passwd:/etc/passwd:ro -e FALCO_VERSION=${FALCO_VERSION} -v /home/user/src::/source -v /home/user/build/falco:/build falcosecurity/falco-tester
+```
+
 ## Build directly on host
 
 If you'd rather build directly on the host, you can use your local toolchain and cmake binaries.