docker: various improvements to Dockerfile

This includes the following changes:

- Use multi-stage build to run application in an optimized environment, see
  https://docs.docker.com/develop/develop-images/multistage-build/
- Run application on alpine instead of archlinux to further reduce image size
- Build Crystal application with --release for improved runtime performance
- Run application as non-root user for better security, see
  https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#user
- Only rebuild Docker layers when required

docker/Dockerfile

@@ -1,15 +1,28 @@
-FROM archlinux/base
-
-RUN pacman -Sy --noconfirm shards crystal imagemagick librsvg \
-    which pkgconf gcc ttf-liberation glibc
-# base-devel contains many other basic packages, that are normally assumed to already exist on a clean arch system
-
-ADD . /invidious
-
+FROM alpine:latest AS builder
+RUN apk add -u crystal shards libc-dev \
+    yaml-dev libxml2-dev sqlite-dev sqlite-static zlib-dev openssl-dev
 WORKDIR /invidious
+COPY ./shard.yml ./shard.yml
+RUN shards update && shards install
+COPY ./src/ ./src/
+# TODO: .git folder is required for building – this is destructive.
+# See definition of CURRENT_BRANCH, CURRENT_COMMIT and CURRENT_VERSION.
+COPY ./.git/ ./.git/
+RUN crystal build --static --release \
+# TODO: Remove next line, see https://github.com/crystal-lang/crystal/issues/7946
+    -Dmusl \
+    ./src/invidious.cr
 
-RUN sed -i 's/host: localhost/host: postgres/' config/config.yml && \
-    shards update && shards install && \
-    crystal build src/invidious.cr
-
+FROM alpine:latest
+RUN apk add -u imagemagick ttf-opensans
+WORKDIR /invidious
+RUN addgroup -g 1000 -S invidious && \
+    adduser -u 1000 -S invidious -G invidious
+COPY ./assets/ ./assets/
+COPY ./config/config.yml ./config/config.yml
+COPY ./config/sql/ ./config/sql/
+COPY ./locales/ ./locales/
+RUN sed -i 's/host: localhost/host: postgres/' config/config.yml
+COPY --from=builder /invidious/invidious .
+USER invidious
 CMD [ "/invidious/invidious" ]