docker: refactor and update docker image build

[smikes]

Fixes tailscale/tailscale#15674

See https://github.com/smikes/tailscale-android/tree/smikes/simplify-docker-expanded for step-by-step refactoring of dockerfile

update docker image datestamp
make docker-shell target use the build image with an explicit /bin/bash command retain the docker-remove-shell-image target in case something depends on it, but clarify that it is now a no-op
hoist all ENV declarations
hoist all mkdir commands
combine mkdir commands
use env vars in mkdir
combine apt-get commands into single RUN
make argument order consistent in apt-get commands collect packages into fewer apt-get commands
add clean up of temporary files after apt-get
expand apt-get commands to one package per line
sort packages in apt-get commands
combine go installation commands into single RUN
update golang version to 1.24.1
also copy build.gradle file
ensure gradlew is executable before running
remove trailing newline

[review-ai-agent]

Pull Request Revisions

Revision	Description
r5	Dockerfile apt-get commands externalized Extracted Docker build apt-get commands into a separate shell script for improved modularity and maintainability
r4	No changes since last revision
r3	Downgraded Android Gradle plugin version Modified Android Gradle plugin version from 8.11.1 to 8.6.1 in build.gradle configuration
r2	No changes since last revision
r1	Updated Android build and Docker configurations Modified Makefile, build.gradle, and Docker build configurations, including updating Gradle version, Go version, and consolidating Docker build steps

✅ AI review completed for r5

Help

React with emojis to give feedback on AI-generated reviews:

• 👍 means the feedback was helpful and actionable
• 👎 means the feedback was incorrect or unhelpful

💬 Replying to feedback with a comment helps us improve the system. Your input also contributes to shaping future interactions with the AI reviewer.

We'd love to hear from you—reach out anytime at team@review.ai.

[danderson]

Issuebot wants the issue mentioned in the git commit message, for posterity. The syntax it recognizes is Updates tailscale/tailscale#15674, or Fixes tailscale/tailscale#15674 to close the bug on merge (though not sure if that works across repos, but I can fix it in post if necessary)