Skip to content

Build

wctr build: OCI images from a Containerfile or Dockerfile, for WASI. Build v0.4.0, built into wctr, with shell-form RUN and live step output.

wctr build builds OCI images from a Containerfile (or a Dockerfile, same syntax) for the engine. It is the separate module go-wasmcontainers/build (v0.4.0), which drives the engine through its API, the way BuildKit sits beside a container daemon: each step runs in a container made from the previous step’s image, which is then committed. Pure Go, so it also runs in the browser.

wctr build is built into wctr since cmd/wctr v0.1.0 (see Getting started); the standalone wctr-build still exists. It needs a wctrd of v0.12.0 or later, and v0.17.0 or later for shell-form RUN.

$ wctr build -t lab/app:v1 .
$ wctr run --rm lab/app:v1

Flags

wctr build [OPTIONS] PATH | -

Flag
-t, --tag NAME[:TAG]name the image (repeatable)
-f, --file FILEthe Containerfile (-: standard input); default PATH/Containerfile, else PATH/Dockerfile
--build-arg K=Va build argument (K alone: from the environment)
--target STAGEbuild up to this stage
--no-cacherun every step
--progress plainplain progress, the only kind
-q, --quietprint only the image ID

PATH is the build context, sent without what its .containerignore (else .dockerignore) excludes; - reads the context as a tar on standard input. Multi-stage builds work.

What works, and what does not yet

  • Exec-form RUN ["prog", "arg"] works: it runs a WebAssembly program in the step’s container.
  • Shell-form RUN cmd works on an image with a shell (build v0.3.0). It runs /bin/sh through the engine’s WASIX process subset (engine v0.17.0 or later), so pipes, redirections, && and command substitution work, with coreutils on the shell base image (wctr pull ghcr.io/go-wasmcontainers/shell:0.1.0, public). In an image without a runnable shell the build stops with a precise error.
  • A step’s output streams as the step runs (build v0.4.0), instead of being printed when it ends.
  • Not supported, each a precise error and never ignored: here-documents, RUN --mount, ADD from a URL or Git, COPY --parents/--exclude, --chown by name, symbolic --chmod, and xz/zstd archives in ADD.

Cache

A step is reused when its parent image, the configuration it produces, the instruction and the digest of its inputs are the same. The cache is kept in $XDG_CACHE_HOME/wasmcontainers/build-cache.json (in memory in a browser).

Speed

A seven-instruction build with one RUN took 61.7 ms cold and 4.8 ms with every step cached (build v0.2.0). Measured under heavy load (a load average of 66), so indicative only, and not yet compared with BuildKit.

Created on by tannevaled

Modified on by tannevaled

NetworkingLimitations