Docker image doesn't run on Alpine 3.19

[Travis Ralston]

Released a few days ago: https://www.alpinelinux.org/posts/Alpine-3.19.0-released.html

Both golang:1.20-alpine and alpine:latest default to 3.19 - forcing to 3.18 appears to allow the binary to run again.

Note that the build succeeds on either version. It's the runtime that has an issue (with libheif symbols, specifically).