Fix container image build (#178)

Fixes #177
Use python:3.9-alpine as base to reduce reliance on alpine maintainers
for python and remove already provided packages (python3, python3-dev,
ca-certificates).
Remove bash since there is already a shell provided and the
entrypoint-script is not using bash-exclusive syntax.
Change entrypoint-script's shebang from bash to sh & fix
array-to-string assignment see https://github.com/koalaman/shellcheck/wiki/SC2124

Dockerfile

@@ -1,12 +1,10 @@
-FROM alpine:3.8
-
-RUN set -x && \
-    apk add --no-cache bash python3 ca-certificates postgresql-libs postgresql-dev && \
-    apk add --no-cache --virtual=build-dependencies build-base python3-dev && \
-    pip3 install --upgrade --no-cache-dir pip && \
-    pip3 install --no-cache-dir psycopg2-binary migra && \
-    apk del build-dependencies postgresql-dev python3-dev && \
-    rm -rf /tmp/* /var/tmp/* /var/cache/apk/*
+FROM python:3.9-alpine
+
+RUN apk add --update --no-cache --upgrade postgresql-libs && \
+  apk add --no-cache --virtual=build-dependencies build-base postgresql-dev && \
+  pip install --no-cache-dir packaging psycopg2-binary migra && \
+  apk del build-dependencies && \
+  rm -rf /tmp/* /var/tmp/* /var/cache/apk/*
 
 COPY docker-entrypoint.sh /docker-entrypoint.sh
 

docker-entrypoint.sh

@@ -1,11 +1,10 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
 set -e
 
 if [ "$1" = 'migra' ]; then
 	if [ "${MIGRA_LOG_COMMAND}" = 'true' ]; then
-		migra_command="$@"
-		echo "${migra_command}"
+		echo "$*"
 	fi
 fi