adjust supercronic download to allow multiarch builds

[guidopetri]

The download of supercronic in the Dockerfile hardcodes amd64 as the architecture, but we can make it flexible to allow for multiarch builds.

For this purpose I added all three of the SHA1SUMs and just select the "correct" one to use based on the ARCH build argument (which has a default of amd64).

This built successfully for me with the command:

docker build -t stringer-arch-test --build-arg ARCH=arm64 .

(as well as arm and amd64)

I changed the shell that executes the Dockerfile to bash to take advantage of the ${!ENV_VAR_CONTAINING_OTHER_ENV_VAR_NAME} construction - the SUPERCRONIC_SHA1SUM arg contains one of SUPERCRONIC_{amd64,arm,arm64}_SHA1SUM, and we need to evaluate this value as an env var. The easiest way to do this with /bin/sh seems to be eval echo \$$SUPERCRONIC_SHA1SUM, but that's a bit unsafe since we're using eval so I selected to switch to bash instead.

[guidopetri]

@mockdeep / @hozza I think this is ready for review :)

[mockdeep]

This all looks pretty reasonable to me. Since we've moved to GoodJob, we might eventually be able to use their repeating jobs support, but I think it makes sense to get this working for now.

@hozza do you want to take a look before I merge this?

[mockdeep]

thanks @guidopetri!