Preserve empty continuation lines (backslash-only separator lines) in RUN #59

Problem

dockerfmt removes empty continuation lines — lines containing only a backslash (\) — that are used as visual separators between logical groups inside long RUN instructions:

FROM alpine
RUN \
  --mount=type=bind,source=./a,target=/b \
  \
  echo one \
  \
  && echo two \
  \
  && echo three

is formatted to:

FROM alpine
RUN --mount=type=bind,source=./a,target=/b \
    echo one \
    && echo two \
    && echo three

All three separator lines are gone.

These lines are valid Dockerfile syntax (a continuation line with empty content). No linter complains about them: BuildKit's NoEmptyContinuation build check only fires on completely blank lines inside a continuation, and hadolint has no rule for them. So removing them is a pure formatting-style loss, and users currently have no way to keep them short of # dockerfmt-ignore on the whole directive.

Root cause

formatRun hands the RUN body to mvdan/sh (formatBash → syntax.Printer), and the mvdan/sh printer only preserves empty continuation lines in some positions. Probes with plain shfmt -bn:

The third case is what bites after extractDirectiveContent splits the --mount flags off the body: any separator that followed the flags becomes a leading empty continuation and is dropped. Separators before && lines are dropped by the printer as well.

Suggested fix

dockerfmt already has a placeholder mechanism that lets comments survive the shfmt pass (preprocessShellComments / postprocessShellComments wrap comment lines in backtick placeholders and unwrap them afterwards). The same trick could be applied to \-only lines: temporarily replace each empty continuation line with a placeholder comment line (which the printer preserves in all positions), then unwrap it back into a bare \ after printing. That would preserve the separators everywhere, including before && lines and right after the flags.

Related: comments in the flags region are dropped

A manual workaround for the above is to use comment lines as separators. In the command region this works well and round-trips:

RUN \
  --mount=type=bind,source=./a,target=/b \
  # --- section
  echo one \
  # --- next
  && echo two

However, comments placed before or between RUN flags are silently removed, together with the leading RUN \ continuation:

FROM alpine
RUN \
  # \
  --mount=type=bind,source=./a,target=/b \
  --mount=type=cache,target=/c \
  echo one

→ (single pass):

FROM alpine
RUN --mount=type=bind,source=./a,target=/b \
    --mount=type=cache,target=/c \
    echo one

The same happens with a comment placed between two --mount flags. Since comments in the command region are preserved (and re-indented — a great feature), it would be consistent to preserve them in the flags region too.