docker: fix build on armv7

Signed-off-by: László Várady <laszlo.varady@anno.io>

Author: MrAnno

docker/axosyslog-builder.dockerfile

@@ -35,6 +35,8 @@ RUN apk add --update-cache \
       alpine-sdk \
       sudo \
       git \
+      # Rust is needed on armv7 for https://cryptography.io/en/46.0.4/installation/#rust
+      rust \
     && apk upgrade -a \
     && adduser -D builder \
     && addgroup builder abuild \