Merge "Update floss containerized build docs"

image necessary to build Floss. If building a new Podman image, run

`podman-build-image.py` with the tag `floss:latest`.

## Setting up podman

On most Debian based machines, you should be able to simply use apt-get and

install these requisite packages.

```

sudo apt-get install \

  podman \

  uidmap

```

Then, we need to set up podman for [rootless

mode](https://docs.podman.io/en/latest/markdown/podman.1.html#rootless-mode):

```

sudo usermod --add-subuids 10000-75535 $USERNAME

sudo usermod --add-subgids 10000-75535 $USERNAME

```

## Generating the flossbuild image

Run the following to generate the required image:

```

podman-build-image.py --tag floss:latest

```

This uses the default tag of `flossbuild:latest` so you don't have to provide it

specifically when invoking `build-in-podman.py`.

## Using the podman image to build

Once the Podman image is built (and assuming it's tagged as `floss:latest`), you

should use the `build-in-podman.py` script to build the current repo.

Basic build:

```

build-in-podman.py

```

This script will use the local `floss:latest` (or pull it from the registry),

mount (or create) the `floss-out` volume to `/root/.floss` and the current

source to `/root/src` before running these commands in the container: