tally/consistent-indentation¶

Enforces consistent indentation for Dockerfile build stages.

Description¶

Enforces consistent indentation to visually separate build stages in multi-stage Dockerfiles. This rule always uses tabs for indentation.

Behavior depends on the number of stages:

• Multi-stage (2+ FROM instructions): Commands within each stage must be indented with 1 tab. FROM lines remain at column 0.
• Single-stage (1 FROM instruction): All indentation is removed — tabs, spaces, or any mix. Since there is no stage structure to communicate, indenting commands adds noise. The auto-fix strips all leading whitespace from every instruction.

Why tabs only?¶

Docker heredoc syntax (<<-) strips leading tabs from body lines. Spaces have no equivalent shell whitespace treatment — using them for indentation would corrupt heredoc content when <<- is applied. Because this rule must convert << to <<- when adding indentation to heredoc instructions, only tabs produce correct results.

FROM alpine:3.20
    COPY <<-EOF /etc/config
        key=value
        other=setting
    EOF

With spaces, <<- cannot strip indentation, so the content would retain unwanted leading whitespace.

Multi-stage (indentation required)¶

FROM golang:1.23 AS builder
    WORKDIR /src
    COPY . .
    RUN go build -o /app

FROM alpine:3.20
    COPY --from=builder /app /usr/local/bin/app
    ENTRYPOINT ["app"]

Single-stage (no indentation)¶

FROM alpine:3.20
RUN apk add --no-cache curl
COPY . /app
CMD ["./app"]

Examples¶

Bad (multi-stage without indentation)¶

FROM golang:1.23 AS builder
WORKDIR /src
RUN go build -o /app
FROM alpine:3.20
COPY --from=builder /app /app

Good (multi-stage with tab indentation)¶

FROM golang:1.23 AS builder
    WORKDIR /src
    RUN go build -o /app
FROM alpine:3.20
    COPY --from=builder /app /app

Bad (single-stage with indentation)¶

In a single-stage Dockerfile, indentation is unnecessary and will be removed by --fix:

# Before (violation: unexpected indentation)
FROM alpine:3.20
    RUN apk add curl
    COPY . /app

# After --fix (indentation removed)
FROM alpine:3.20
RUN apk add curl
COPY . /app

Configuration¶

Enable the rule (no configurable options — tabs are always used):

[rules.tally.consistent-indentation]
severity = "style"

Auto-fix¶

This rule provides safe auto-fixes that adjust indentation:

• Multi-stage: Adds 1 tab indentation to commands within stages
• Single-stage: Removes all leading whitespace (tabs and spaces) from commands
• Style correction: Replaces wrong indent characters (e.g., spaces to tabs)
• Heredoc <<- conversion: When tab indentation is applied to a heredoc instruction (RUN <<EOF, COPY <<EOF), the fix converts << to <<- so that BuildKit strips the leading tabs from the heredoc body

tally lint --fix Dockerfile