Dockerfile: cleanup.

Remove some stuff now https://github.com/Homebrew/homebrew-linux-dev/pull/203 is merged.

Dockerfile

@@ -4,10 +4,7 @@ FROM ${img}
 ARG img
 
 ENV HOMEBREW_FORCE_HOMEBREW_ON_LINUX=1
-ENV HOMEBREW_ON_DEBIAN7=1
 ENV HOMEBREW_DEVELOPER=1
-ENV HOMEBREW_CURL_PATH=/usr/bin/curl
-ENV HOMEBREW_GIT_PATH=/usr/bin/git
 ENV HOMEBREW_NO_AUTO_UPDATE=1
 
 # Needs to run as root for GitHub Actions
@@ -16,9 +13,4 @@ USER root
 
 # Don't want to pin package versions
 # hadolint ignore=DL3008
-RUN apt-get update && \
-    apt-get install -y --no-install-recommends \
-       bzip2 && \
-    apt-get clean && \
-    rm -rf /var/lib/apt/lists/* && \
-    mkdir -p /home/linuxbrew/.linuxbrew/Homebrew/Library/Taps/homebrew/homebrew-portable
+RUN mkdir -p /home/linuxbrew/.linuxbrew/Homebrew/Library/Taps/homebrew/homebrew-portable