What causes "invalid Dockerfile" in slimtoolkit/slim?

Verify the file at the given path exists, is non-empty, and is actually a Dockerfile (starts with a valid instruction like FROM).

slimtoolkit/slim · error

invalid Dockerfile

Error message

invalid Dockerfile

What it means

ErrInvalidDockerfile is a sentinel error in the dockerfile parser package indicating the Dockerfile input could not be parsed as a valid Dockerfile. It is returned by FromFile when parsing a Dockerfile from disk fails because the file is empty, unreadable, or structurally invalid. The parser cannot extract any usable instruction stream from it.

Source

Thrown at pkg/docker/dockerfile/parser/parser.go:17

// Package parser implements a Dockerfile parser
package parser

import (
	"errors"
	"os"
	"path/filepath"
	"strconv"
	"strings"

	"github.com/slimtoolkit/slim/pkg/docker/dockerfile/ast"
	"github.com/slimtoolkit/slim/pkg/docker/dockerfile/spec"
	"github.com/slimtoolkit/slim/pkg/docker/instruction"
)

var (
	ErrInvalidDockerfile = errors.New("invalid Dockerfile")
)

//TODO:
//* support incremental, partial and instruction level parsing
//* support parsing from reader and from string

func FromFile(fpath string) (*spec.Dockerfile, error) {
	fo, err := os.Open(fpath)
	if err != nil {
		return nil, err
	}

	defer fo.Close()

	astParsed, err := ast.Parse(fo)
	if err != nil {
		return nil, err
	}

View on GitHub (pinned to 81940d17fa)

Solutions

Verify the file at the given path exists, is non-empty, and is actually a Dockerfile (starts with a valid instruction like FROM).
Run 'docker build' or a linter on the same Dockerfile to confirm syntax validity independently.
Check that your tooling passes the Dockerfile path, not the build-context directory, to FromFile.
Regenerate or restore the Dockerfile from version control if it was truncated or corrupted.

Example fix

// before
ast, err := parser.FromFile(ctx, "./build")
// after
ast, err := parser.FromFile(ctx, "./Dockerfile")

Defensive patterns

Strategy: validation

Validate before calling

info, err := os.Stat(path)
if err != nil || info.IsDir() || info.Size() == 0 {
    return fmt.Errorf("dockerfile missing or empty: %s", path)
}
// optional: sniff first line for FROM/ARG/escape directive

Try / catch

if _, err := parser.FromFile(ctx, path); err != nil {
    if errors.Is(err, parser.ErrInvalidDockerfile) {
        // surface path + first bytes for diagnosis
    }
    return err
}

Prevention

Always pass a file path, not a directory, to the parser.
Verify the file is non-empty before parsing.
Validate Dockerfile syntax in CI with a linter or dry-run build.
Restore Dockerfiles from VCS rather than generating ad hoc.

When it happens

Trigger: Calling dockerfile/parser FromFile with a path that points to an empty file, a non-Dockerfile text file, or a file that fails AST-level parsing.

Common situations: Pointing build tooling at the wrong path (e.g. a directory or a .dockerignore), a Dockerfile deleted or truncated by CI, or a Dockerfile with unsupported syntax from a newer builder version.

Related errors

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/978a9d10ce35c23b. Report an issue: GitHub.