refactor: move dockerfiles to package folders

gateway/Dockerfile

@@ -0,0 +1,22 @@
+# twilight requires newer rustc than what is in alpine:latest
+FROM alpine:edge AS builder
+
+RUN apk add cargo protobuf
+
+# Precache crates.io index
+RUN cargo search >/dev/null
+
+WORKDIR /build
+COPY proto/ /build/proto
+COPY gateway/ /build/gateway
+COPY myriad_rs/ /build/myriad_rs
+
+# todo: cache build of myriad_rs elsewhere
+
+RUN (cd gateway && cargo build --release)
+
+FROM alpine:latest
+
+COPY --from=builder /build/gateway/target/release/pluralkit /opt/gateway
+
+ENTRYPOINT ["/opt/gateway"]