Dockerfile: work around a ShellCheck error

ShellCheck sees Dockerfile as a regular shell script, so it cannot
understand it when the function definition `retry()` comes after the
`RUN` command (it sees it as a single command `RUN retry() ...`). As
this causes a parsing error, it cannot be worked around with a
`# shellcheck disable` comment. Instead, let's add a no-op `:` command
before the function definition, so that ShellCheck can happily parse the
file.