Makefile: add RUNC_BUILDTAGS, deprecate EXTRA_BUILDTAGS

A bit of history. EXTRA_BUILDTAGS was introduced in commit dac417174,
as a quick way to add some extra Go build tags to the runc build.

Later, commit 767bc008 changed Makefile to not get EXTRA_TAGS from the
shell environment, as the name is quite generic and some unrelated
environment variable with that name can affect runc build. While such
change does make sense, it makes it more complicated to pass build tags
in CI and otherwise (see e.g. commit 0e1fe368a).

Moreover, runc build uses some Go build tags by default (via Makefile),
and while it is easy to add more build tags (via EXTRA_BUILDTAGS), in
order to remove some existing tags one has to redefine BUILDTAGS from
scratch, which is not very convenient (again, see commit 0e1fe368a which
gets the current value of BUILDTAGS from the Makefile in order to remove
a single tag).

To handle all of the above, let's do this:
 - implement RUNC_BUILDTAGS, fixing the issue of not-so-unique name;
 - allow to get RUNC_BUILDTAGS from shell environment;
 - implement a feature to remove a build tag from default set by
   prefixing it with "-" (as in RUNC_BUILDTAGS="-seccomp");
 - document all this in README;
 - make CI use the new feature;
 - keep EXTRA_BUILDTAGS for backward compatibility, add a make warning
   and a TODO to remove it for runc 1.6.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/test.yml

@@ -84,12 +84,10 @@ jobs:
       run: |
         sudo -E PATH="$PATH" ./script/build-libpathrs.sh "$LIBPATHRS_VERSION" /usr
 
-    - name: configure custom BUILDTAGS
+    - name: remove libpathrs build tag
       if: ${{ matrix.libpathrs == '' }}
-      run: |-
-        # Strip out libpathrs from the default buildtags.
-        CUSTOM_BUILDTAGS="$(make -pn | sed -En '/^BUILDTAGS :?=/ { s/.*=// ; s/\blibpathrs\b//p }')"
-        echo "CUSTOM_BUILDTAGS=$CUSTOM_BUILDTAGS" >>"$GITHUB_ENV"
+      run: |
+        echo RUNC_BUILDTAGS=-libpathrs >>"$GITHUB_ENV"
 
     - name: install CRIU
       if: ${{ matrix.criu == '' }}
@@ -121,11 +119,7 @@ jobs:
         check-latest: true
 
     - name: build
-      run: |
-        sudo -E PATH="$PATH" make \
-            ${CUSTOM_BUILDTAGS:+BUILDTAGS="$CUSTOM_BUILDTAGS"} \
-            EXTRA_FLAGS="${{ matrix.race }}" \
-            all
+      run: sudo -E PATH="$PATH" make EXTRA_FLAGS="${{ matrix.race }}" all
 
     - name: Setup Bats and bats libs
       uses: bats-core/bats-action@4.0.0
@@ -144,7 +138,7 @@ jobs:
 
     - name: unit test
       if: matrix.rootless != 'rootless'
-      run: sudo -E PATH="$PATH" -- make ${CUSTOM_BUILDTAGS:+BUILDTAGS="$CUSTOM_BUILDTAGS"} TESTFLAGS="${{ matrix.race }}" localunittest
+      run: sudo -E PATH="$PATH" -- make TESTFLAGS="${{ matrix.race }}" localunittest
 
     - name: add rootless user
       if: matrix.rootless == 'rootless'
@@ -154,7 +148,7 @@ jobs:
 
     - name: integration test (fs driver)
       continue-on-error: ${{ matrix.criu != '' }} # Don't let criu-dev errors fail CI.
-      run: sudo -E PATH="$PATH" script -e -c 'make ${CUSTOM_BUILDTAGS:+BUILDTAGS="$CUSTOM_BUILDTAGS"} local${{ matrix.rootless }}integration'
+      run: sudo -E PATH="$PATH" script -e -c 'make local${{ matrix.rootless }}integration'
 
     - name: integration test (systemd driver)
       continue-on-error: ${{ matrix.criu != '' }} # Don't let criu-dev errors fail CI.
@@ -165,7 +159,7 @@ jobs:
         printf "[Service]\nDelegate=yes\n" | sudo tee /etc/systemd/system/user@.service.d/delegate.conf
         sudo systemctl daemon-reload
         # Run the tests.
-        sudo -E PATH="$PATH" script -e -c 'make RUNC_USE_SYSTEMD=yes ${CUSTOM_BUILDTAGS:+BUILDTAGS="$CUSTOM_BUILDTAGS"} local${{ matrix.rootless }}integration'
+        sudo -E PATH="$PATH" script -e -c 'make RUNC_USE_SYSTEMD=yes local${{ matrix.rootless }}integration'
 
   # We need to continue support for 32-bit ARM.
   # However, we do not have 32-bit ARM CI, so we use i386 for testing 32bit stuff.