Before this change, `go get`-ting` would pick the latest version of `shfmt` which keeps changing, hence frequently breaking our builds.
After this change, we download the `shfmt` binary from its GitHub releases page which is fast and locks the version we use. Note that `RUN` does not like herestrings, hence `echo "..." | sha256sum -c` for checksum.
Alternatives considered:
- `go get gopkg.in/mvdan/sh.v1/cmd/shfmt` -- but `gopkg.in` currently doesn't behave well for this tool
- `go build` from a `git` tag -- but this is much more involved.
Marc CARRE
d4a98a58f4