Update Dockerfile template to match best practices

Changes:

- Remove `-y` flag from `apt-get update`, as it
  has no effect on that command.
- Use `--no-install-recommends` flag to avoid
  installing unnecessary dependencies.
- Use more general wildcard to remove apt package
  list cache.
- Remove `apt-get clean` command, as official
  Debian and Ubuntu images run it automatically.
- Update example in the documentation to match
  the changes.

Reference:

- https://docs.docker.com/build/building/best-practices/#apt-get

guides/deployment/releases.md

@@ -181,8 +181,8 @@ ARG RUNNER_IMAGE="debian:${DEBIAN_VERSION}"
 FROM ${BUILDER_IMAGE} AS builder
 
 # install build dependencies
-RUN apt-get update -y && apt-get install -y build-essential git \
-    && apt-get clean && rm -f /var/lib/apt/lists/*_*
+RUN apt-get update && apt-get install -y --no-install-recommends build-essential git \
+    && rm -rf /var/lib/apt/lists/*
 
 # prepare build dir
 WORKDIR /app
@@ -227,9 +227,9 @@ RUN mix release
 # the compiled release and other runtime necessities
 FROM ${RUNNER_IMAGE}
 
-RUN apt-get update -y && \
-  apt-get install -y libstdc++6 openssl libncurses5 locales ca-certificates \
-  && apt-get clean && rm -f /var/lib/apt/lists/*_*
+RUN apt-get update && \
+  apt-get install -y --no-install-recommends libstdc++6 openssl libncurses5 locales ca-certificates \
+  && rm -rf /var/lib/apt/lists/*
 
 # Set the locale
 RUN sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && locale-gen

priv/templates/phx.gen.release/Dockerfile.eex

@@ -21,8 +21,8 @@ ARG RUNNER_IMAGE="debian:${DEBIAN_VERSION}"
 FROM ${BUILDER_IMAGE} AS builder
 
 # install build dependencies
-RUN apt-get update -y && apt-get install -y build-essential git \
-    && apt-get clean && rm -f /var/lib/apt/lists/*_*
+RUN apt-get update && apt-get install -y --no-install-recommends build-essential git \
+    && rm -rf /var/lib/apt/lists/*
 
 # prepare build dir
 WORKDIR /app
@@ -69,9 +69,9 @@ RUN mix release
 # the compiled release and other runtime necessities
 FROM ${RUNNER_IMAGE}
 
-RUN apt-get update -y && \
-  apt-get install -y libstdc++6 openssl libncurses5 locales ca-certificates \
-  && apt-get clean && rm -f /var/lib/apt/lists/*_*
+RUN apt-get update && \
+  apt-get install -y --no-install-recommends libstdc++6 openssl libncurses5 locales ca-certificates \
+  && rm -rf /var/lib/apt/lists/*
 
 # Set the locale
 RUN sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && locale-gen