tink-cli: Add bash to container image

I've recently been bitten by alpine/busybox's ash not properly implementing
`[[` as a builtin while doing a bunch of work on sandbox's scripts. Changing
over to POSIX sh is an option but not really worth it imo as almost everyone
assumes sh == bash already and bash is also nice for interactive use anyway.

Bash ends up adding ~2.2MB to the image which I don't think matters, tink-cli
isn't going to be used in production all that much and its still pretty small.

tink-cli-yes-bash                latest            3f59f8e81a34   10 seconds ago   30.4MB
tink-cli-no-bash                 latest            e521b3895111   23 seconds ago   28.2MB

Signed-off-by: Manuel Mendez <mmendez@equinix.com>

cmd/tink-cli/Dockerfile

@@ -4,5 +4,5 @@ CMD sleep infinity
 ARG TARGETARCH
 ARG TARGETVARIANT
 
-RUN apk add --no-cache --update --upgrade ca-certificates
+RUN apk add --no-cache --update --upgrade bash ca-certificates
 COPY tink-cli-linux-${TARGETARCH:-amd64}${TARGETVARIANT} /usr/bin/tink