Refactor Dockerfile: consolidate instructions

Dockerfile

@@ -1,22 +1,37 @@
-FROM ubuntu:20.04
+# syntax=docker/dockerfile:1.3
 
-RUN apt-get update && apt-get install -y \
-    ca-certificates \
-    locales \
-    python3 \
-    python3-setuptools
-RUN localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
-RUN mkdir /usr/src/app
-COPY setup.cfg /usr/src/app
-COPY setup.py /usr/src/app
-COPY *.md /usr/src/app/
+FROM docker.io/library/ubuntu:20.04
+
+ENV DEBIAN_FRONTEND="noninteractive"
+
+RUN apt-get update \
+    && apt-get install -y \
+        ca-certificates \
+        locales \
+        python3 \
+        python3-setuptools \
+    && localedef \
+        -i en_US \
+        -c \
+        -f UTF-8 \
+        -A /usr/share/locale/locale.alias en_US.UTF-8
+
+COPY setup.cfg setup.py *.md /usr/src/app/
 COPY doc/*.md /usr/src/app/doc/
 COPY man/asciinema.1 /usr/src/app/man/
-COPY asciinema /usr/src/app/asciinema
+COPY asciinema/ /usr/src/app/asciinema/
+
 WORKDIR /usr/src/app
+
 RUN python3 setup.py install
-ENV LANG en_US.utf8
-ENV SHELL /bin/bash
-ENV USER docker
+
+ENV LANG="en_US.utf8"
+ENV SHELL="/bin/bash"
+ENV USER="docker"
+
 WORKDIR /root
+
 ENTRYPOINT ["/usr/local/bin/asciinema"]
+CMD ["--help"]
+
+# vim:ft=dockerfile