Building multiarch docker images

Hi!

I'm having issues building multi-arch Docker image with supercronic.

I'm able to use TARGETARCH in URL and binary name, but not in the sha1 checksum variable.

Anyone know how to deal with this?

Could we solve this with having a separate file with .sha1 extension so that you can get the checksum value from URL.

```
ENV SUPERCRONIC_URL=https://github.com/aptible/supercronic/releases/download/v0.2.29/supercronic-linux-${TARGETARCH} \
SUPERCRONIC=supercronic-linux-${TARGETARCH} \
SUPERCRONIC_SHA1SUM=${SUPERCRONIC_URL}.sha1

RUN curl -fsSLO "$SUPERCRONIC_URL" \
&& curl -fsSLO "$SUPERCRONIC_SHA1SUM" ~
&& sha1sum -c ${SUPERCRONIC}.sha1 \
&& chmod +x "$SUPERCRONIC" \
&& mv "$SUPERCRONIC" "/usr/local/bin/${SUPERCRONIC}" \
&& ln -s "/usr/local/bin/${SUPERCRONIC}" /usr/local/bin/supercronic
```

Start by inspecting the repository's multi-architecture Docker build configuration and the release checksum assets described in the issue. Verify how TARGETARCH is used for the binary and checksum, then confirm that a multi-architecture build can download the matching checksum and complete sha1 verification.

Written by the indexing model from the issue text.