Build custom configuration

cartesi-rollups/development/building-the-application.md

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 ```
+
+## Custom Configuration
+
+The build process of an application is customizable by providing a configuration file called `cartesi.toml` located at the working directory. An alternative file can also be specified with the `--config` build option.
+
+If no file is defined the implicit default configuration is as follows:
+
+```toml
+[drives.root]
+builder = "docker"
+dockerfile = "Dockerfile"
+format = "ext2"
+```
+
+This defines a single drive called `root`, using the `docker` builder. The `docker` builder uses Docker to build a drive by executing `docker buildx build` with the `Dockerfile`, and then creating an `ext2` drive from the Docker image contents.
+
+## Drives
+
+A Cartesi machine is compose by a set of drives. Each drive is defined by a name and a builder. The builder is responsible for creating the drive contents. The `root` drive is the only required one.
+
+The following builders are available: `directory`, `docker`, `empty`, `none`, `tar`.
+
+## The `docker` builder
+
+The `docker` builder is the most common builder and is used to build drives from Docker images.
+
+```toml
+[drives.root]
+builder = "docker"
+dockerfile = "Dockerfile"
+format = "ext2"
+extraSize = "100Mb"
+```
+
+The configuration above builds a Docker image using the specified `dockerfile`, and then creates an `ext2` drive with the image contents.
+
+The `extraSize` attribute adds extra free space to the drive in additional to the size of the image. It can be specified as a number in bytes or as a string with a number followed by a unit (e.g. `100Mb`). Supported units and abbreviations are as follows and are case-insensitive:
+
+- `b` for bytes
+- `kb` for kilobytes
+- `mb` for megabytes
+- `gb` for gigabytes
+- `tb` for terabytes
+- `pb` for petabytes
+
+The `format` attribute specifies the drive format and can be `ext2` or `sqfs`. SquashFS drives are compressed and read-only.
+
+Instead of building a Docker image it's also possible to use a pre-existing Docker image (or a manually built image) by specifying a `image` attribute with the image name or ID. This way the user can use Docker directly to build, and all its features like tagging, multi-stage builds, etc, and then use `cartesi build` to create the Cartesi machine.
+
+## The `directory` builder
+
+The `directory` builder creates a drive with the contents of a directory.
+
+```toml
+[drives.data]
+builder = "directory"
+directory = "./data"
+extraSize = "100Mb"
+format = "ext2"
+```
+
+The `directory` is the only required attribute and specifies the path of the drive contents.
+The `extraSize` attribute adds extra free space to the drive in additional to the size of the contents. It can be specified as a number in bytes or as a string with a number followed by a unit (e.g. `100Mb`). Supported units and abbreviations are the same as for the `extraSize` attribute of the `docker` builder.
+
+## The `empty` builder
+
+The `empty` builder creates an empty drive with a specific size. This is generally used to create drives that will hold application data, separate from other drives that hold the application itself or system files.
+
+```toml
+[drives.data]
+builder = "empty"
+size = "100Mb"
+```
+
+The only required attribute is `size`, which specifies the drive size. It can be specified as a number in bytes or as a string with a number followed by a unit (e.g. `100Mb`). Supported units and abbreviations are the same as for the `extraSize` attribute of the `directory` builder.
+
+## The `none` builder
+
+The `none` builder is a no-op builder, meaning that it defines a drive with a pre-existing image, built externally.
+
+```toml
+[drives.root]
+builder = "none"
+filename = "./rootfs-tools-v0.15.0.ext2"
+```
+
+The only required attribute is `filename` which specifies the path of the drive image. The format of the drive is inferred from the file extension.
+
+## The `tar` builder
+
+This builder is very similar to the `directory` builder, except that it creates a drive from a tarball file.
+
+```toml
+[drives.data]
+builder = "tar"
+filename = "build/files.tar"
+```
+
+Instead of specifying a `directory` attribute, the `tar` builder requires a `filename` attribute with the path of the tarball file. The `format` and `extraSize` are also supported.
+
+## Cartesi Machine
+
+The configuration file also allows modifying the Cartesi machine booting procedure. The following attribute are available:
+
+```toml
+[machine]
+assert_rolling_update = true
+bootargs = ["no4lvl", "quiet", "earlycon=sbi", "console=hvc0", "rootfstype=ext2", "root=/dev/pmem0", "rw", "init=/usr/sbin/cartesi-init"]
+entrypoint = "/usr/local/bin/app"
+max-mcycle = 0
+no-rollup = false
+ram-image = "/usr/share/cartesi-machine/images/linux.bin" # directory inside SDK image
+ram-length = "128Mi"
+```
+
+The above example changes the default behavior of the boot to not run any cycles by specifying `max-mcycle = 0`.
+The semantics of each attribute can be found in the [Cartesi Machine documentation](https://docs.cartesi.io/cartesi-machine/host/cmdline/).