Extend `asdf` to support musl compiled crystal

[caspiano]

To build portable binaries, asdf must be extended to fetch crystal binaries built against musl instead of glibc.

[stakach]

can be achieved with this command

docker buildx build --platform linux/amd64,linux/arm64 --output type=local,dest=folder .

will output static musl executables to the filesystem with a dockerfile that looks like

FROM alpine:3.16 as build
WORKDIR /app

# Create a non-privileged user, defaults are appuser:10001
ARG IMAGE_UID="10001"
ENV UID=$IMAGE_UID
ENV USER=appuser

# See https://stackoverflow.com/a/55757473/12429735
RUN adduser \
    --disabled-password \
    --gecos "" \
    --home "/nonexistent" \
    --shell "/sbin/nologin" \
    --no-create-home \
    --uid "${UID}" \
    "${USER}"

# Add trusted CAs for communicating with external services
RUN apk add --no-cache \
        ca-certificates \
    && \
    update-ca-certificates

# Add crystal lang
# can look up packages here: https://pkgs.alpinelinux.org/packages?name=crystal
RUN apk add \
  --update \
  --no-cache \
  --repository=http://dl-cdn.alpinelinux.org/alpine/edge/main \
  --repository=http://dl-cdn.alpinelinux.org/alpine/edge/community \
    crystal \
    shards \
    yaml-dev \
    yaml-static \
    libxml2-dev \
    openssl-dev \
    openssl-libs-static \
    zlib-dev \
    zlib-static \
    tzdata \
    libssh2-dev \
    libssh2-static

# Install any additional dependencies
# RUN apk add libssh2 libssh2-dev

# Install shards for caching
COPY shard.yml shard.yml
COPY shard.override.yml shard.override.yml
COPY shard.lock shard.lock

RUN shards install --production --ignore-crystal-version --skip-postinstall --skip-executables

# Add src
COPY ./src /app/src

# Build application
ARG COMPILE_DRIVER="src/place/meet.cr"
ENV DRIVER_FILE=$COMPILE_DRIVER
RUN crystal build --release --debug --error-trace --static "${DRIVER_FILE}" -o driver
SHELL ["/bin/ash", "-eo", "pipefail", "-c"]

# Extract binary dependencies (uncomment if not compiling a static build)
# RUN for binary in /app/bin/*; do \
#        ldd "$binary" | \
#        tr -s '[:blank:]' '\n' | \
#        grep '^/' | \
#        xargs -I % sh -c 'mkdir -p $(dirname deps%); cp % deps%;'; \
#      done

# Build a minimal docker image
FROM scratch
WORKDIR /
ENV PATH=$PATH:/

# Copy the user information over
COPY --from=build etc/passwd /etc/passwd
COPY --from=build /etc/group /etc/group

# These are required for communicating with external services
COPY --from=build /etc/hosts /etc/hosts

# These provide certificate chain validation where communicating with external services over TLS
COPY --from=build /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
ENV SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt

# This is required for Timezone support
COPY --from=build /usr/share/zoneinfo/ /usr/share/zoneinfo/

# This is your application
# COPY --from=build /app/deps / # uncomment if not compiling a static build
COPY --from=build /app/driver /driver

# Use an unprivileged user.
USER appuser:appuser
CMD ["/driver"]