String substitution in variable expansion

[tstenner]

One of the variable expansion functions missing from #4287 was string substition, so I implemented it.

Motivating example:

ARG VERSION=1.2.3
ARG URL=https://github.com/foo-corp/foo/releases/download/$VERSION/foo-$VERSION-${TARGETARCH}.tar.gz
ADD ${URL//arm64/aarch64} /tmp/foo.tar.gz
RUN tar -xf /tmp/foo.tar.gz

The two parts following the variable name (pattern and replacement) need to be parsed separately with processStopWords, otherwise variable expansion could cause a / in a variable referenced in the pattern to be interpreted as separator between patern and replacement. This means that the second / is required (i.e. ${foo/bar/} to remove bar, compared to ${foo/bar} as accepted by various shells).

On the positive side, wildcards and slashes in referenced variables are handled correctly (e.g. PAT='*/'; REPL='.../'; ${FOO/$PAT/$REPL}).

[dvdksn]

Awesome! Docs LGTM :-)

[tstenner]

Surprisingly, all CI checks passed even though I added more tests.

There's one decision left: referencing an undefined variable is not an error (as it is in bash without set -u), but personally I feel it should be.

[tonistiigi]

PTAL @jsternberg

[jsternberg]

Sorry for taking so long to get to this. I've had it on my backburner for awhile. I'll have a review for this by the end of my current day.

[jsternberg]

LGTM