Dockerfile: trim GCC installation by 52 MB (#66)

This commit reduces our image size by about 50 MB by deleting some
large, unnecessary files.

Image size before this commit: 139.68 MB
Image size with this commit:    87.48 MB

The majority of the image size comes from the GCC installation, but we
don't need things like LTO support or the D standard library in our
image.

Therefore this commit removes these large files:
21.5M    /usr/libexec/gcc/x86_64-alpine-linux-musl/10.2.1/lto1
21.5M    /usr/bin/lto-dump
5.8M     /usr/lib/libgphobos.so.1.0.0
976.0K   /usr/libexec/gcc/x86_64-alpine-linux-musl/10.2.1/lto-wrapper

The largest remaining files in the image:
$ find / -type f -exec du -k {} + | sort -nr | head -n20 | cut -f2 | xargs du -sh
22.4M    /usr/libexec/gcc/x86_64-alpine-linux-musl/10.2.1/cc1
9.4M     /usr/lib/libc.a
4.6M     /nim/bin/nim
2.9M     /usr/lib/gcc/x86_64-alpine-linux-musl/10.2.1/libgcc.a
2.6M     /usr/lib/libmpfr.so.6.1.0
2.5M     /lib/libcrypto.so.1.1
1.6M     /usr/lib/libstdc++.so.6.0.28
1.6M     /usr/lib/libisl.so.22.0.0
1.6M     /usr/bin/ld
1.5M     /usr/bin/dwp
1.3M     /usr/lib/libopcodes-2.35.2.so
1.2M     /usr/lib/gcc/x86_64-alpine-linux-musl/10.2.1/plugin/gtype.state
1.1M     /usr/lib/libgdruntime.so.1.0.0
1.1M     /usr/lib/libbfd-2.35.2.so
1.1M     /usr/bin/gcc
1.1M     /usr/bin/cpp
952.0K   /usr/lib/gcc/x86_64-alpine-linux-musl/10.2.1/plugin/include/insn-flags.h
808.0K   /bin/busybox
604.0K   /usr/bin/as
592.0K   /lib/ld-musl-x86_64.so.1

The final disk usage in the root directory:
$ du -k -d1 / | sort -nr | cut -f2 | xargs du -sh
87.9M    /
67.6M    /usr
9.9M     /nim
3.9M     /lib
1.1M     /bin
428.0K   /etc
324.0K   /sbin
12.0K    /var
0        /tmp
0        /srv
0        /run
0        /root
0        /opt
0        /mnt
0        /media
0        /home

Note that:
- The `busybox` `sort` available in the image from doesn't have the `-h`
  option, hence the above convoluted piping.
- We can't delete the files by force-removing dependencies of the `gcc`
  package. That is, something like `apk del --force libgphobos` doesn't
  work.

Dockerfile

@@ -24,12 +24,18 @@ COPY src/runner.nim /build/
 COPY src/unittest_json.nim /build/
 RUN /nim/bin/nim c -d:release -d:lto -d:strip /build/runner.nim
 
-FROM base
+FROM ${REPO}:${IMAGE}
 COPY --from=nim_builder /nim/ /nim/
 # hadolint ignore=DL3018
 RUN apk add --no-cache \
+      gcc \
+      musl-dev \
       pcre \
-    && ln -s /nim/bin/nim /usr/local/bin/nim
+    && ln -s /nim/bin/nim /usr/local/bin/nim \
+    && printf '\nRemoving some unneeded large files:\n' \
+    && rm -v /usr/bin/lto-dump \
+    && find / -path '/usr/libexec/gcc/x86_64-alpine-linux-musl/*/lto*' -exec rm -v {} + \
+    && find / -path '/usr/lib/libgphobos.so*' -exec rm -v {} +
 WORKDIR /opt/test-runner/
 COPY --from=runner_builder /build/runner bin/
 COPY bin/run.sh bin/